postconf(5) - Man pages

POSTCONF(5) File Formats Manual POSTCONF(5)

NAME
postconf - Postfix configuration parameters

SYNOPSIS
postconf parameter ...

postconf -e "parameter=value" ...

DESCRIPTION
The Postfix main.cf configuration file specifies parameters that control
the operation of the Postfix mail system. Typically the file contains
only a small subset of all parameters; parameters not specified are left
at their default values.

The general format of the main.cf file is as follows:

• Each logical line has the form "parameter = value". Whitespace
around the "=" is ignored, as is whitespace at the end of a logi-
cal line.

• Empty lines and whitespace-only lines are ignored, as are lines
whose first non-whitespace character is a `#'.

• A logical line starts with non-whitespace text. A line that
starts with whitespace continues a logical line.

• A parameter value may refer to other parameters.

• The expressions "$name" and "${name}" are recursively re-
placed with the value of the named parameter. The parame-
ter name must contain only characters from the set [a-zA-
Z0-9_]. An undefined parameter value is replaced with the
empty value.

• The expressions "${name?value}" and "${name?{value}}" are
replaced with "value" when "$name" is non-empty. The para-
meter name must contain only characters from the set [a-
zA-Z0-9_]. These forms are supported with Postfix versions
>= 2.2 and >= 3.0, respectively.

• The expressions "${name:value}" and "${name:{value}}" are
replaced with "value" when "$name" is empty. The parameter
name must contain only characters from the set [a-zA-
Z0-9_]. These forms are supported with Postfix versions >=
2.2 and >= 3.0, respectively.

• The expression "${name?{value1}:{value2}}" is replaced
with "value1" when "$name" is non-empty, and with "value2"
when "$name" is empty. The "{}" is required for "value1",
optional for "value2". The parameter name must contain
only characters from the set [a-zA-Z0-9_]. This form is
supported with Postfix versions >= 3.0.

• The first item inside "${...}" may be a relational expres-
sion of the form: "{value3} == {value4}". Besides the "=="
(equality) operator Postfix supports "!=" (inequality),
"<", "<=", ">=", and ">". The comparison is numerical when
both operands are all digits, otherwise the comparison is
lexicographical. These forms are supported with Postfix
versions >= 3.0.

• Each "value" is subject to recursive named parameter and
relational expression evaluation, except where noted.

• Whitespace before or after each "{value}" is ignored.

• Specify "$$" to produce a single "$" character.

• The legacy form "$(...)" is equivalent to the preferred
form "${...}".

• When the same parameter is defined multiple times, only the last
instance is remembered.

• Otherwise, the order of main.cf parameter definitions does not
matter.

The remainder of this document is a description of all Postfix configu-
ration parameters. Default values are shown after the parameter name in
parentheses, and can be looked up with the "postconf -d" command.

Note: this is not an invitation to make changes to Postfix configuration
parameters. Unnecessary changes can impair the operation of the mail
system.

2bounce_notice_recipient (default: postmaster)
The recipient of undeliverable mail that cannot be returned to the
sender. This feature is enabled with the notify_classes parameter.

access_map_defer_code (default: 450)
The numerical Postfix SMTP server response code for an access(5) map
"defer" action, including "defer_if_permit" or "defer_if_reject". Prior
to Postfix 2.6, the response is hard-coded as "450".

Do not change this unless you have a complete understanding of RFC 5321.

This feature is available in Postfix 2.6 and later.

access_map_reject_code (default: 554)
The numerical Postfix SMTP server response code for an access(5) map
"reject" action.

Do not change this unless you have a complete understanding of RFC 5321.

address_verify_cache_cleanup_interval (default: 12h)
The amount of time between verify(8) address verification database
cleanup runs. This feature requires that the database supports the
"delete" and "sequence" operators. Specify a zero interval to disable
database cleanup.

After each database cleanup run, the verify(8) daemon logs the number of
entries that were retained and dropped. A cleanup run is logged as "par-
tial" when the daemon terminates early after "postfix reload", "postfix
stop", or no requests for $max_idle seconds.

Specify a non-negative time value (an integral value plus an optional
one-letter suffix that specifies the time unit). Time units: s (sec-
onds), m (minutes), h (hours), d (days), w (weeks). The default time
unit is h (hours).

This feature is available in Postfix 2.7.

address_verify_default_transport (default: $default_transport)
Overrides the default_transport parameter setting for address verifica-
tion probes.

This feature is available in Postfix 2.1 and later.

address_verify_local_transport (default: $local_transport)
Overrides the local_transport parameter setting for address verification
probes.

This feature is available in Postfix 2.1 and later.

address_verify_map (default: see postconf -d output)
Lookup table for persistent address verification status storage. The
table is maintained by the verify(8) service, and is opened before the
process releases privileges.

The lookup table is persistent by default (Postfix 2.7 and later).
Specify an empty table name to keep the information in volatile memory
which is lost after "postfix reload" or "postfix stop". This is the de-
fault with Postfix version 2.6 and earlier.

Specify a location in a file system that will not fill up. If the data-
base becomes corrupted, the world comes to an end. To recover, delete
(NOT: truncate) the file and do "postfix reload".

Postfix daemon processes do not use root privileges when opening this
file (Postfix 2.5 and later). The file must therefore be stored under a
Postfix-owned directory such as the data_directory. As a migration aid,
an attempt to open the file under a non-Postfix directory is redirected
to the Postfix-owned data_directory, and a warning is logged.

Examples:

address_verify_map = hash:/var/lib/postfix/verify
address_verify_map = btree:/var/lib/postfix/verify

This feature is available in Postfix 2.1 and later.

address_verify_negative_cache (default: yes)
Enable caching of failed address verification probe results. When this
feature is enabled, the cache may pollute quickly with garbage. When
this feature is disabled, Postfix will generate an address probe for
every lookup.

This feature is available in Postfix 2.1 and later.

address_verify_negative_expire_time (default: 3d)
The time after which a failed probe expires from the address verifica-
tion cache.

Specify a non-zero time value (an integral value plus an optional
one-letter suffix that specifies the time unit). Time units: s (sec-
onds), m (minutes), h (hours), d (days), w (weeks). The default time
unit is d (days).

This feature is available in Postfix 2.1 and later.

address_verify_negative_refresh_time (default: 3h)
The time after which a failed address verification probe needs to be re-
freshed.

This feature is available in Postfix 2.1 and later.

address_verify_pending_request_limit (default: see postconf -d output)
A safety limit that prevents address verification requests from over-
whelming the Postfix queue. By default, the number of pending requests
is limited to 1/4 of the active queue maximum size (qmgr_message_ac-
tive_limit). The queue manager enforces the limit by tempfailing re-
quests that exceed the limit. This affects only unknown addresses and
inactive addresses that have expired, because the verify(8) daemon auto-
matically refreshes an active address before it expires.

This feature is available in Postfix 3.1 and later.

address_verify_poll_count (default: normal: 3, overload: 1)
How many times to query the verify(8) service for the completion of an
address verification request in progress.

By default, the Postfix SMTP server polls the verify(8) service up to
three times under non-overload conditions, and only once when under
overload. With Postfix version 2.5 and earlier, the SMTP server always
polls the verify(8) service up to three times by default.

Specify 1 to implement a crude form of greylisting, that is, always de-
fer the first delivery request for a new address.

Examples:

# Postfix <= 2.6 default
address_verify_poll_count = 3
# Poor man's greylisting
address_verify_poll_count = 1

This feature is available in Postfix 2.1 and later.

address_verify_poll_delay (default: 3s)
The delay between queries for the completion of an address verification
request in progress.

The default polling delay is 3 seconds.

This feature is available in Postfix 2.1 and later.

address_verify_positive_expire_time (default: 31d)
The time after which a successful probe expires from the address verifi-
cation cache.

This feature is available in Postfix 2.1 and later.

address_verify_positive_refresh_time (default: 7d)
The time after which a successful address verification probe needs to be
refreshed. The address verification status is not updated when the
probe fails (optimistic caching).

This feature is available in Postfix 2.1 and later.

address_verify_relay_transport (default: $relay_transport)
Overrides the relay_transport parameter setting for address verification
probes.

This feature is available in Postfix 2.1 and later.

address_verify_relayhost (default: $relayhost)
Overrides the relayhost parameter setting for address verification
probes. This information can be overruled with the transport(5) table.

This feature is available in Postfix 2.1 and later.

address_verify_sender (default: $double_bounce_sender)
The sender address to use in address verification probes; prior to Post-
fix 2.5 the default was "postmaster". To avoid problems with address
probes that are sent in response to address probes, the Postfix SMTP
server excludes the probe sender address from all SMTPD access blocks.

Specify an empty value (address_verify_sender =) or <> if you want to
use the null sender address. Beware, some sites reject mail from <>,
even though RFCs require that such addresses be accepted.

Examples:

address_verify_sender = <>
address_verify_sender = postmaster@mydomain

This feature is available in Postfix 2.1 and later.

address_verify_sender_dependent_default_transport_maps (default: $sender_depen-
dent_default_transport_maps)
Overrides the sender_dependent_default_transport_maps parameter setting
for address verification probes.

This feature is available in Postfix 2.7 and later.

address_verify_sender_dependent_relayhost_maps (default: $sender_dependent_re-
layhost_maps)
Overrides the sender_dependent_relayhost_maps parameter setting for ad-
dress verification probes.

This feature is available in Postfix 2.3 and later.

address_verify_sender_ttl (default: 0s)
The time between changes in the time-dependent portion of address veri-
fication probe sender addresses. The time-dependent portion is appended
to the localpart of the address specified with the address_verify_sender
parameter. This feature is ignored when the probe sender addresses is
the null sender, i.e. the address_verify_sender value is empty or <>.

Historically, the probe sender address was fixed. This has caused such
addresses to end up on spammer mailing lists, and has resulted in wasted
network and processing resources.

To enable time-dependent probe sender addresses, specify a non-zero time
value. Specify a value of at least several hours, to avoid problems with
senders that use greylisting. Avoid nice TTL values, to make the result
less predictable.

This feature is available in Postfix 2.9 and later.

address_verify_service_name (default: verify)
The name of the verify(8) address verification service. This service
maintains the status of sender and/or recipient address verification
probes, and generates probes on request by other Postfix processes.

address_verify_transport_maps (default: $transport_maps)
Overrides the transport_maps parameter setting for address verification
probes.

This feature is available in Postfix 2.1 and later.

address_verify_virtual_transport (default: $virtual_transport)
Overrides the virtual_transport parameter setting for address verifica-
tion probes.

This feature is available in Postfix 2.1 and later.

alias_database (default: see postconf -d output)
The alias databases for local(8) delivery that are updated with
"newaliases" or with "sendmail -bi".

This is a separate configuration parameter because not all the tables
specified with $alias_maps have to be local files.

Examples:

alias_database = hash:/etc/aliases
alias_database = hash:/etc/mail/aliases

alias_maps (default: see postconf -d output)
Optional lookup tables that are searched only with an email address lo-
calpart (no domain) and that apply only to local(8) recipients; this is
unlike virtual_alias_maps that are often searched with a full email ad-
dress (including domain) and that apply to all recipients: local(8),
virtual, and remote. The alias_maps table format and lookups are docu-
mented in aliases(5). For an overview of Postfix address manipulations
see the ADDRESS_REWRITING_README document.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found. Note: these lookups are recursive.

The default list is system dependent. On systems with NIS, the default
is to search the local alias database, then the NIS alias database.

If you change the alias database, run "postalias /etc/aliases" (or wher-
ever your system stores the mail alias file), or simply run "newaliases"
to build the necessary DBM or DB file.

The local(8) delivery agent disallows regular expression substitution of
$1 etc. in alias_maps, because that would open a security hole.

The local(8) delivery agent will silently ignore requests to use the
proxymap(8) server within alias_maps. Instead it will open the table di-
rectly. Before Postfix version 2.2, the local(8) delivery agent will
terminate with a fatal error.

Examples:

alias_maps = hash:/etc/aliases, nis:mail.aliases
alias_maps = hash:/etc/aliases

allow_mail_to_commands (default: alias, forward)
Restrict local(8) mail delivery to external commands. The default is to
disallow delivery to "|command" in :include: files (see aliases(5) for
the text that defines this terminology).

Specify zero or more of: alias, forward or include, in order to allow
commands in aliases(5), .forward files or in :include: files, respec-
tively.

Example:

allow_mail_to_commands = alias,forward,include

allow_mail_to_files (default: alias, forward)
Restrict local(8) mail delivery to external files. The default is to
disallow "/file/name" destinations in :include: files (see aliases(5)
for the text that defines this terminology).

Specify zero or more of: alias, forward or include, in order to allow
"/file/name" destinations in aliases(5), .forward files and in :include:
files, respectively.

Example:

allow_mail_to_files = alias,forward,include

allow_min_user (default: no)
Allow a sender or recipient address to have `-' as the first character.
By default, this is not allowed, to avoid accidents with software that
passes email addresses via the command line. Such software would not be
able to distinguish a malicious address from a bona fide command-line
option. Although this can be prevented by inserting a "--" option termi-
nator into the command line, this is difficult to enforce consistently
and globally.

As of Postfix version 2.5, this feature is implemented by triv-
ial-rewrite(8). With earlier versions this feature was implemented by
qmgr(8) and was limited to recipient addresses only.

allow_percent_hack (default: yes)
Enable the rewriting of the form "user%domain" to "user@domain". This
is enabled by default.

Note: as of Postfix version 2.2, message header address rewriting hap-
pens only when one of the following conditions is true:

• The message is received with the Postfix sendmail(1) command,

• The message is received from a network client that matches $lo-
cal_header_rewrite_clients,

• The message is received from the network, and the re-
mote_header_rewrite_domain parameter specifies a non-empty value.

To get the behavior before Postfix version 2.2, specify "lo-
cal_header_rewrite_clients = static:all".

Example:

allow_percent_hack = no

allow_srv_lookup_fallback (default: no)
When SRV record lookup fails or no SRV record exists, fall back to MX or
IP address lookup as if SRV record lookup was not enabled.

This feature is available in Postfix 3.8 and later.

allow_untrusted_routing (default: no)
Forward mail with sender-specified routing (user[@%!]remote[@%!]site)
from untrusted clients to destinations matching $relay_domains.

By default, this feature is turned off. This closes a nasty open relay
loophole where a backup MX host can be tricked into forwarding junk mail
to a primary MX host which then spams it out to the world.

This parameter also controls if non-local addresses with sender-speci-
fied routing can match Postfix access tables. By default, such addresses
cannot match Postfix access tables, because the address is ambiguous.

alternate_config_directories (default: empty)
A list of non-default Postfix configuration directories that may be
specified with "-c config_directory" on the command line (in the case of
sendmail(1), with the "-C" option), or via the MAIL_CONFIG environment
parameter.

This list must be specified in the default Postfix main.cf file, and
will be used by set-gid Postfix commands such as postqueue(1) and post-
drop(1).

Specify absolute pathnames, separated by comma or space. Note: $name ex-
pansion is not supported.

always_add_missing_headers (default: no)
Always add (Resent-) From:, To:, Date: or Message-ID: headers when not
present. Postfix 2.6 and later add these headers only when clients
match the local_header_rewrite_clients parameter setting. Earlier Post-
fix versions always add these headers; this may break DKIM signatures
that cover non-existent headers. The undisclosed_recipients_header pa-
rameter setting determines whether a To: header will be added.

always_bcc (default: empty)
Optional address that receives a "blind carbon copy" of each message
that is received by the Postfix mail system.

Note: with Postfix 2.3 and later the BCC address is added as if it was
specified with NOTIFY=NONE. The sender will not be notified when the BCC
address is undeliverable, as long as all down-stream software implements
RFC 3461.

Note: with Postfix 2.2 and earlier the sender will be notified when the
BCC address is undeliverable.

Note: automatic BCC recipients are produced only for new mail. To avoid
mailer loops, automatic BCC recipients are not generated after Postfix
forwards mail internally, or after Postfix generates mail itself.

Note: automatic BCC recipients are subject to address canonicalization
(add missing domain), canonical_maps, masquerade_domains, and vir-
tual_alias_maps.

anvil_rate_time_unit (default: 60s)
The time unit over which client connection rates and other rates are
calculated.

This feature is implemented by the anvil(8) service which is available
in Postfix version 2.2 and later.

The default interval is relatively short. Because of the high frequency
of updates, the anvil(8) server uses volatile memory only. Thus, infor-
mation is lost whenever the process terminates.

anvil_status_update_time (default: 600s)
How frequently the anvil(8) connection and rate limiting server logs
peak usage information.

This feature is available in Postfix 2.2 and later.

append_at_myorigin (default: yes)
With locally submitted mail, append the string "@$myorigin" to mail ad-
dresses without domain information. With remotely submitted mail, append
the string "@$remote_header_rewrite_domain" instead.

Note 1: this feature is enabled by default and must not be turned off.
Postfix does not support domain-less addresses.

Note 2: with Postfix version 2.2, message header address rewriting hap-
pens only when one of the following conditions is true:

• The message is received with the Postfix sendmail(1) command,

• The message is received from a network client that matches $lo-
cal_header_rewrite_clients,

• The message is received from the network, and the re-
mote_header_rewrite_domain parameter specifies a non-empty value.

To get the behavior before Postfix version 2.2, specify "lo-
cal_header_rewrite_clients = static:all".

append_dot_mydomain (default: Postfix >= 3.0: no, Postfix < 3.0: yes)
With locally submitted mail, append the string ".$mydomain" to addresses
that have no ".domain" information. With remotely submitted mail, append
the string ".$remote_header_rewrite_domain" instead.

Note 1: When disabled (Postfix 3.0 and later), users will not be able to
send mail to "user@partialdomainname" but will have to specify full do-
main names instead.

Note 2: with Postfix version 2.2, message header address rewriting hap-
pens only when one of the following conditions is true:

• The message is received with the Postfix sendmail(1) command,

• The message is received from a network client that matches $lo-
cal_header_rewrite_clients,

• The message is received from the network, and the re-
mote_header_rewrite_domain parameter specifies a non-empty value.

To get the behavior before Postfix version 2.2, specify "lo-
cal_header_rewrite_clients = static:all".

application_event_drain_time (default: 100s)
How long the postkick(1) command waits for a request to enter the Post-
fix daemon process input buffer before giving up.

This feature is available in Postfix 2.1 and later.

authorized_flush_users (default: static:anyone)
List of users who are authorized to flush the queue.

By default, all users are allowed to flush the queue. Access is always
granted if the invoking user is the super-user or the $mail_owner user.
Otherwise, the real UID of the process is looked up in the system pass-
word file, and access is granted only if the corresponding login name is
on the access list. The username "unknown" is used for processes whose
real UID is not found in the password file.

Specify a list of user names, "/file/name" or "type:table" patterns,
separated by commas and/or whitespace. The list is matched left to
right, and the search stops on the first match. A "/file/name" pattern
is replaced by its contents; a "type:table" lookup table is matched when
a name matches a lookup key (the lookup result is ignored). Continue
long lines by starting the next line with whitespace. Specify "!pattern"
to exclude a name from the list. The form "!/file/name" is supported
only in Postfix version 2.4 and later.

This feature is available in Postfix 2.2 and later.

authorized_mailq_users (default: static:anyone)
List of users who are authorized to view the queue.

By default, all users are allowed to view the queue. Access is always
granted if the invoking user is the super-user or the $mail_owner user.
Otherwise, the real UID of the process is looked up in the system pass-
word file, and access is granted only if the corresponding login name is
on the access list. The username "unknown" is used for processes whose
real UID is not found in the password file.

Specify a list of user names, "/file/name" or "type:table" patterns,
separated by commas and/or whitespace. The list is matched left to
right, and the search stops on the first match. A "/file/name" pattern
is replaced by its contents; a "type:table" lookup table is matched when
a name matches a lookup key (the lookup result is ignored). Continue
long lines by starting the next line with whitespace. Specify "!pattern"
to exclude a user name from the list. The form "!/file/name" is sup-
ported only in Postfix version 2.4 and later.

This feature is available in Postfix 2.2 and later.

authorized_submit_users (default: static:anyone)
List of users who are authorized to submit mail with the sendmail(1)
command (and with the privileged postdrop(1) helper command).

By default, all users are allowed to submit mail. Otherwise, the real
UID of the process is looked up in the system password file, and access
is granted only if the corresponding login name is on the access list.
The username "unknown" is used for processes whose real UID is not found
in the password file. To deny mail submission access to all users spec-
ify an empty list.

Specify a list of user names, "/file/name" or "type:table" patterns,
separated by commas and/or whitespace. The list is matched left to
right, and the search stops on the first match. A "/file/name" pattern
is replaced by its contents; a "type:table" lookup table is matched when
a name matches a lookup key (the lookup result is ignored). Continue
long lines by starting the next line with whitespace. Specify "!pattern"
to exclude a user name from the list. The form "!/file/name" is sup-
ported only in Postfix version 2.4 and later.

Example:

authorized_submit_users = !www, static:all

This feature is available in Postfix 2.2 and later.

authorized_verp_clients (default: $mynetworks)
What remote SMTP clients are allowed to specify the XVERP command. This
command requests that mail be delivered one recipient at a time with a
per recipient return address.

By default, only trusted clients are allowed to specify XVERP.

This parameter was introduced with Postfix version 1.1. Postfix version
2.1 renamed this parameter to smtpd_authorized_verp_clients and changed
the default to none.

Specify a list of network/netmask patterns, separated by commas and/or
whitespace. The mask specifies the number of bits in the network part of
a host address. You can also specify hostnames or .domain names (the
initial dot causes the domain to match any name below it), "/file/name"
or "type:table" patterns. A "/file/name" pattern is replaced by its
contents; a "type:table" lookup table is matched when a table entry
matches a lookup string (the lookup result is ignored). Continue long
lines by starting the next line with whitespace. Specify "!pattern" to
exclude an address or network block from the list. The form
"!/file/name" is supported only in Postfix version 2.4 and later.

Note: IP version 6 address information must be specified inside [] in
the authorized_verp_clients value, and in files specified with
"/file/name". IP version 6 addresses contain the ":" character, and
would otherwise be confused with a "type:table" pattern.

backwards_bounce_logfile_compatibility (default: yes)
Produce additional bounce(8) logfile records that can be read by Postfix
versions before 2.0. The current and more extensible "name = value" for-
mat is needed in order to implement more sophisticated functionality.

This feature is available in Postfix 2.1 and later.

berkeley_db_create_buffer_size (default: 16777216)
The per-table I/O buffer size for programs that create Berkeley DB hash
or btree tables. Specify a byte count.

This feature is available in Postfix 2.0 and later.

berkeley_db_read_buffer_size (default: 131072)
The per-table I/O buffer size for programs that read Berkeley DB hash or
btree tables. Specify a byte count.

This feature is available in Postfix 2.0 and later.

best_mx_transport (default: empty)
Where the Postfix SMTP client should deliver mail when it detects a
"mail loops back to myself" error condition. This happens when the local
MTA is the best SMTP mail exchanger for a destination not listed in $my-
destination, $inet_interfaces, $proxy_interfaces, $virtual_alias_do-
mains, or $virtual_mailbox_domains. By default, the Postfix SMTP client
returns such mail as undeliverable.

Specify, for example, "best_mx_transport = local" to pass the mail from
the Postfix SMTP client to the local(8) delivery agent. You can specify
any message delivery "transport" or "transport:nexthop" that is defined
in the master.cf file. See the transport(5) manual page for the syntax
and meaning of "transport" or "transport:nexthop".

However, this feature is expensive because it ties up a Postfix SMTP
client process while the local(8) delivery agent is doing its work. It
is more efficient (for Postfix) to list all hosted domains in a table or
database.

biff (default: yes)
Whether or not to use the local biff service. This service sends "new
mail" notifications to users who have requested new mail notification
with the UNIX command "biff y".

For compatibility reasons this feature is on by default. On systems
with lots of interactive users, the biff service can be a performance
drain. Specify "biff = no" in main.cf to disable.

body_checks (default: empty)
Optional lookup tables for content inspection as specified in the
body_checks(5) manual page.

Note: with Postfix versions before 2.0, these rules inspect all content
after the primary message headers.

body_checks_size_limit (default: 51200)
How much text in a message body segment (or attachment, if you prefer to
use that term) is subjected to body_checks inspection. The amount of
text is limited to avoid scanning huge attachments.

This feature is available in Postfix 2.0 and later.

bounce_notice_recipient (default: postmaster)
The recipient of postmaster notifications with the message headers of
mail that Postfix did not deliver and of SMTP conversation transcripts
of mail that Postfix did not receive. This feature is enabled with the
notify_classes parameter.

bounce_queue_lifetime (default: 5d)
Consider a bounce message as undeliverable, when delivery fails with a
temporary error, and the time in the queue has reached the
bounce_queue_lifetime limit. By default, this limit is the same as for
regular mail.

Specify 0 when mail delivery should be tried only once.

This feature is available in Postfix 2.1 and later.

bounce_service_name (default: bounce)
The name of the bounce(8) service. This service maintains a record of
failed delivery attempts and generates non-delivery notifications.

This feature is available in Postfix 2.0 and later.

bounce_size_limit (default: 50000)
The maximal amount of original message text that is sent in a non-deliv-
ery notification. Specify a byte count. A message is returned as either
message/rfc822 (the complete original) or as text/rfc822-headers (the
headers only). With Postfix version 2.4 and earlier, a message is al-
ways returned as message/rfc822 and is truncated when it exceeds the
size limit.

Notes:

• If you increase this limit, then you should increase the
mime_nesting_limit value proportionally.

• Be careful when making changes. Excessively large values will
result in the loss of non-delivery notifications, when a bounce
message size exceeds a local or remote MTA's message size limit.

bounce_template_file (default: empty)
Pathname of a configuration file with bounce message templates. These
override the built-in templates of delivery status notification (DSN)
messages for undeliverable mail, delayed mail, successful delivery, or
delivery verification. The bounce(5) manual page describes how to edit
and test template files.

Template message body text may contain $name references to Postfix con-
figuration parameters. The result of $name expansion can be previewed
with "postconf -b file_name" before the file is placed into the Postfix
configuration directory.

This feature is available in Postfix 2.3 and later.

broken_sasl_auth_clients (default: no)
Enable interoperability with remote SMTP clients that implement an obso-
lete version of the AUTH command (RFC 4954). Examples of such clients
are MicroSoft Outlook Express version 4 and MicroSoft Exchange version
5.0.

Specify "broken_sasl_auth_clients = yes" to have Postfix advertise AUTH
support in a non-standard way.

canonical_classes (default: envelope_sender, envelope_recipient, header_sender,
header_recipient)
What addresses are subject to canonical_maps address mapping. By de-
fault, canonical_maps address mapping is applied to envelope sender and
recipient addresses, and to header sender and header recipient ad-
dresses.

Specify one or more of: envelope_sender, envelope_recipient,
header_sender, header_recipient

This feature is available in Postfix 2.2 and later.

canonical_maps (default: empty)
Optional address mapping lookup tables for message headers and en-
velopes. The mapping is applied to both sender and recipient addresses,
in both envelopes and in headers, as controlled with the canoni-
cal_classes parameter. This is typically used to clean up dirty ad-
dresses from legacy mail systems, or to replace login names by First-
name.Lastname. The table format and lookups are documented in canoni-
cal(5). For an overview of Postfix address manipulations see the AD-
DRESS_REWRITING_README document.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found. Note: these lookups are recursive.

If you use this feature, run "postmap /etc/postfix/canonical" to build
the necessary DBM or DB file after every change. The changes will become
visible after a minute or so. Use "postfix reload" to eliminate the de-
lay.

Note: with Postfix version 2.2, message header address mapping happens
only when message header address rewriting is enabled:

• The message is received with the Postfix sendmail(1) command,

• The message is received from a network client that matches $lo-
cal_header_rewrite_clients,

• The message is received from the network, and the re-
mote_header_rewrite_domain parameter specifies a non-empty value.

To get the behavior before Postfix version 2.2, specify "lo-
cal_header_rewrite_clients = static:all".

Examples:

canonical_maps = dbm:/etc/postfix/canonical
canonical_maps = hash:/etc/postfix/canonical

cleanup_replace_stray_cr_lf (default: yes)
Replace each stray <CR> or <LF> character in message content with a
space character, to prevent outbound SMTP smuggling, and to make the
evaluation of Postfix-added DKIM or other signatures independent from
how a remote mail server handles such characters.

SMTP does not allow such characters unless they are part of a <CR><LF>
sequence, and different mail systems handle such stray characters in an
implementation-dependent manner. Stray <CR> or <LF> characters could be
used for outbound SMTP smuggling, where an attacker uses a Postfix
server to send message content with a non-standard End-of-DATA sequence
that triggers inbound SMTP smuggling at a remote SMTP server.

The replacement happens before all other content management, and before
Postfix may add a DKIM etc. signature; if the signature were created
first, the replacement could invalidate the signature.

In addition to preventing SMTP smuggling, replacing stray <CR> or <LF>
characters ensures that the result of signature validation by later mail
system will not depend on how that mail system handles those stray char-
acters in an implementation-dependent manner.

Note: this feature is disabled with "receive_override_options =
no_header_body_checks".

This feature is available in Postfix >= 3.9, 3.8.5, 3.7.10, 3.6.14, and
3.5.24.

cleanup_service_name (default: cleanup)
The name of the cleanup(8) service. This service rewrites addresses into
the standard form, and performs canonical(5) address mapping and vir-
tual(5) aliasing.

This feature is available in Postfix 2.0 and later.

command_directory (default: see postconf -d output)
The location of all postfix administrative commands.

command_execution_directory (default: empty)
The local(8) delivery agent working directory for delivery to external
commands. Failure to change directory causes the delivery to be de-
ferred.

The command_execution_directory value is not subject to Postfix configu-
ration parameter $name expansion. Instead, the following $name expan-
sions are done on command_execution_directory before the directory is
used. Expansion happens in the context of the delivery request. The re-
sult of $name expansion is filtered with the character set that is spec-
ified with the execution_directory_expansion_filter parameter.

$user The recipient's username.

$shell The recipient's login shell pathname.

$home The recipient's home directory.

$recipient
The full recipient address.

$extension
The optional recipient address extension.

$domain
The recipient domain.

$local The entire recipient localpart.

$recipient_delimiter
The address extension delimiter that was found in the recipient
address (Postfix 2.11 and later), or the system-wide recipient
address extension delimiter (Postfix 2.10 and earlier).

${name?value}

${name?{value}} (Postfix >= 3.0)
Expands to value when $name is non-empty.

${name:value}

${name:{value}} (Postfix >= 3.0)
Expands to value when $name is empty.

${name?{value1}:{value2}} (Postfix >= 3.0)
Expands to value1 when $name is non-empty, value2 otherwise.

Instead of $name you can also specify ${name} or $(name).

This feature is available in Postfix 2.2 and later.

command_expansion_filter (default: see postconf -d output)
Restrict the characters that the local(8) delivery agent allows in $name
expansions of $mailbox_command and $command_execution_directory. Char-
acters outside the allowed set are replaced by underscores.

command_time_limit (default: 1000s)
Time limit for delivery to external commands. This limit is used by the
local(8) delivery agent, and is the default time limit for delivery by
the pipe(8) delivery agent.

Note: if you set this time limit to a large value you must update the
global ipc_timeout parameter as well.

compatibility_level (default: 0)
A safety net that causes Postfix to run with backwards-compatible de-
fault settings after an upgrade to a newer Postfix version.

With backwards compatibility turned on (the main.cf compatibility_level
value is less than the Postfix built-in value), Postfix looks for set-
tings that are left at their implicit default value, and logs a message
when a backwards-compatible default setting is required.

using backwards-compatible default setting name=value
to [accept a specific client request]

using backwards-compatible default setting name=value
to [enable specific Postfix behavior]

See COMPATIBILITY_README for specific message details. If such a message
is logged in the context of a legitimate request, the system administra-
tor should make the backwards-compatible setting permanent in main.cf or
master.cf, for example:

# postconf name=value
# postfix reload

When no more backwards-compatible settings need to be made permanent,
the administrator should turn off backwards compatibility by updating
the compatibility_level setting in main.cf:

# postconf compatibility_level=N
# postfix reload

For N specify the number that is logged in your postfix(1) warning mes-
sage:

warning: To disable backwards compatibility use "postconf
compatibility_level=N" and "postfix reload"

Starting with Postfix version 3.6, the compatibility level in the above
warning message is the Postfix version that introduced the last incom-
patible change. The level is formatted as major.minor.patch, where patch
is usually omitted and defaults to zero. Earlier compatibility levels
are 0, 1 and 2.

NOTE: this also introduces support for the "<level", "<=level", and
other operators to compare compatibility levels. With the standard op-
erators "<", "<=", etc., compatibility level "3.10" would be smaller
than "3.9" which is undesirable.

This feature is available in Postfix 3.0 and later.

config_directory (default: see postconf -d output)
The default location of the Postfix main.cf and master.cf configuration
files. This can be overruled via the following mechanisms:

• The MAIL_CONFIG environment variable (daemon processes and com-
mands).

• The "-c" command-line option (commands only).

With Postfix commands that run with set-gid privileges, a config_direc-
tory override either requires root privileges, or it requires that the
directory is listed with the alternate_config_directories parameter in
the default main.cf file.

confirm_delay_cleared (default: no)
After sending a "your message is delayed" notification, inform the
sender when the delay clears up. This can result in a sudden burst of
notifications at the end of a prolonged network outage, and is therefore
disabled by default.

See also: delay_warning_time.

This feature is available in Postfix 3.0 and later.

connection_cache_protocol_timeout (default: 5s)
Time limit for connection cache connect, send or receive operations.
The time limit is enforced in the client.

This feature is available in Postfix 2.3 and later.

connection_cache_service_name (default: scache)
The name of the scache(8) connection cache service. This service main-
tains a limited pool of cached sessions.

This feature is available in Postfix 2.2 and later.

connection_cache_status_update_time (default: 600s)
How frequently the scache(8) server logs usage statistics with connec-
tion cache hit and miss rates for logical destinations and for physical
endpoints.

connection_cache_ttl_limit (default: 2s)
The maximal time-to-live value that the scache(8) connection cache
server allows. Requests that specify a larger TTL will be stored with
the maximum allowed TTL. The purpose of this additional control is to
protect the infrastructure against careless people. The cache TTL is al-
ready bounded by $max_idle.

content_filter (default: empty)
After the message is queued, send the entire message to the specified
transport:destination. The transport name specifies the first field of a
mail delivery agent definition in master.cf; the syntax of the next-hop
destination is described in the manual page of the corresponding deliv-
ery agent. More information about external content filters is in the
Postfix FILTER_README file.

Notes:

• This setting has lower precedence than a FILTER action that is
specified in an access(5), header_checks(5) or body_checks(5) ta-
ble.

• The meaning of an empty next-hop filter destination is version
dependent. Postfix 2.7 and later will use the recipient domain;
earlier versions will use $myhostname. Specify "default_fil-
ter_nexthop = $myhostname" for compatibility with Postfix 2.6 or
earlier, or specify a content_filter value with an explicit
next-hop destination.

cyrus_sasl_config_path (default: empty)
Search path for Cyrus SASL application configuration files, currently
used only to locate the $smtpd_sasl_path.conf file. Specify zero or
more directories separated by a colon character, or an empty value to
use Cyrus SASL's built-in search path.

Note: some Debian-based Postfix distributions ignore the
"cyrus_sasl_config_path" parameter setting, and force Postfix to open
the file <code>/etc/postfix/sasl/smtpd.conf</code>.

This feature is available in Postfix 2.5 and later when compiled with
Cyrus SASL 2.1.22 or later.

daemon_directory (default: see postconf -d output)
The directory with Postfix support programs and daemon programs. These
should not be invoked directly by humans. The directory must be owned by
root.

daemon_table_open_error_is_fatal (default: no)
How a Postfix daemon process handles errors while opening lookup tables:
gradual degradation or immediate termination.

no (default)
Gradual degradation: a daemon process logs a message of type "er-
ror" and continues execution with reduced functionality. Features
that do not depend on the unavailable table will work normally,
while features that depend on the table will result in a type
"warning" message.
When the notify_classes parameter value contains the "data"
class, the Postfix SMTP server and client will report transcripts
of sessions with an error because a table is unavailable.

yes (historical behavior)
Immediate termination: a daemon process logs a type "fatal" mes-
sage and terminates immediately. This option reduces the number
of possible code paths through Postfix, and may therefore be
slightly more secure than the default.

For the sake of sanity, the number of type "error" messages is limited
to 13 over the lifetime of a daemon process.

This feature is available in Postfix 2.9 and later.

daemon_timeout (default: 18000s)
How much time a Postfix daemon process may take to handle a request be-
fore it is terminated by a built-in watchdog timer.

data_directory (default: see postconf -d output)
The directory with Postfix-writable data files (for example: caches,
pseudo-random numbers). This directory must be owned by the mail_owner
account, and must not be shared with non-Postfix software.

This feature is available in Postfix 2.5 and later.

debug_peer_level (default: 2)
The increment in verbose logging level when a nexthop destination, re-
mote client or server name or network address matches a pattern given
with the debug_peer_list parameter.

Per-nexthop debug logging is available in Postfix 3.6 and later.

debug_peer_list (default: empty)
Optional list of nexthop destination, remote client or server name or
network address patterns that, if matched, cause the verbose logging
level to increase by the amount specified in $debug_peer_level.

Per-nexthop debug logging is available in Postfix 3.6 and later.

Specify domain names, network/netmask patterns, "/file/name" patterns or
"type:table" lookup tables. The right-hand side result from "type:table"
lookups is ignored.

Pattern matching of domain names is controlled by the presence or ab-
sence of "debug_peer_list" in the parent_domain_matches_subdomains para-
meter value.

Examples:

debug_peer_list = 127.0.0.1
debug_peer_list = example.com

debugger_command (default: empty)
The external command to execute when a Postfix daemon program is invoked
with the -D option.

Use "command .. & sleep 5" so that the debugger can attach before the
process marches on. If you use an X-based debugger, be sure to set up
your XAUTHORITY environment variable before starting Postfix.

Note: the command is subject to $name expansion, before it is passed to
the default command interpreter. Specify "$$" to produce a single "$"
character.

Example:

debugger_command =
PATH=/usr/bin:/usr/X11R6/bin
ddd $daemon_directory/$process_name $process_id & sleep 5

default_database_type (default: see postconf -d output)
The default database type for use in newaliases(1), postalias(1) and
postmap(1) commands. On many UNIX systems the default type is either dbm
or hash. The default setting is frozen when the Postfix system is built.

Examples:

default_database_type = hash
default_database_type = dbm

default_delivery_slot_cost (default: 5)
How often the Postfix queue manager's scheduler is allowed to preempt
delivery of one message with another.

Each transport maintains a so-called "available delivery slot counter"
for each message. One message can be preempted by another one when the
other message can be delivered using no more delivery slots (i.e., invo-
cations of delivery agents) than the current message counter has accumu-
lated (or will eventually accumulate - see about slot loans below). This
parameter controls how often the counter is incremented - it happens af-
ter each default_delivery_slot_cost recipients have been delivered.

The cost of 0 is used to disable the preempting scheduling completely.
The minimum value the scheduling algorithm can use is 2 - use it if you
want to maximize the message throughput rate. Although there is no maxi-
mum, it doesn't make much sense to use values above say 50.

The only reason why the value of 2 is not the default is the way this
parameter affects the delivery of mailing-list mail. In the worst case,
delivery can take somewhere between (cost+1/cost) and (cost/cost-1)
times more than if the preemptive scheduler was disabled. The default
value of 5 turns out to provide reasonable message response times while
making sure the mailing-list deliveries are not extended by more than
20-25 percent even in the worst case.

Use transport_delivery_slot_cost to specify a transport-specific over-
ride, where transport is the master.cf name of the message delivery
transport.

Examples:

default_delivery_slot_cost = 0
default_delivery_slot_cost = 2

default_delivery_slot_discount (default: 50)
The default value for transport-specific _delivery_slot_discount set-
tings.

This parameter speeds up the moment when a message preemption can hap-
pen. Instead of waiting until the full amount of delivery slots required
is available, the preemption can happen when transport_deliv-
ery_slot_discount percent of the required amount plus transport_deliv-
ery_slot_loan still remains to be accumulated. Note that the full
amount will still have to be accumulated before another preemption can
take place later.

Use transport_delivery_slot_discount to specify a transport-specific
override, where transport is the master.cf name of the message delivery
transport.

default_delivery_slot_loan (default: 3)
The default value for transport-specific _delivery_slot_loan settings.

Use transport_delivery_slot_loan to specify a transport-specific over-
ride, where transport is the master.cf name of the message delivery
transport.

default_delivery_status_filter (default: empty)
Optional filter in Postfix delivery agents, to replace the delivery sta-
tus code or explanatory text of successful or unsuccessful deliveries.
This allows the replacement of a temporary error status code (4.X.X)
with a permanent error status code (5.X.X) or vice versa, but does not
allow the replacement of a successful status code (2.X.X) with an unsuc-
cessful status code (4.X.X or 5.X.X) or vice versa.

Note: the (smtp|lmtp)_delivery_status_filter is applied only once per
recipient: when delivery is successful, when delivery is rejected with
5XX, or when there are no more alternate MX or A destinations. Use
smtp_reply_filter or lmtp_reply_filter to inspect responses for all de-
livery attempts.

The following parameters can be used to implement a filter for specific
delivery agents: lmtp_delivery_status_filter, local_delivery_status_fil-
ter, pipe_delivery_status_filter, smtp_delivery_status_filter or vir-
tual_delivery_status_filter. These parameters support the same filter
syntax as described here.

Specify zero or more "type:table" lookup table names, separated by comma
or whitespace. For each successful or unsuccessful delivery to a recipi-
ent, the tables are queried in the specified order with one line of text
that is structured as follows:

enhanced-status-code SPACE explanatory-text

The first table match wins. The lookup result must have the same struc-
ture as the query, a successful status code (2.X.X) must be replaced
with a successful status code, an unsuccessful status code (4.X.X or
5.X.X) must be replaced with an unsuccessful status code, and the ex-
planatory text field must be non-empty. Other results will result in a
warning.

Example 1: convert specific soft TLS errors into hard errors, by over-
riding the first number in the enhanced status code.

/etc/postfix/main.cf:
smtp_delivery_status_filter = pcre:/etc/postfix/smtp_dsn_filter

/etc/postfix/smtp_dsn_filter:
/^4(\.\d+\.\d+ TLS is required, but host \S+ refused to start TLS: .+)/
5$1
/^4(\.\d+\.\d+ TLS is required, but was not offered by host .+)/
5$1
# Do not change the following into hard bounces. They may
# result from a local configuration problem.
# 4.\d+.\d+ TLS is required, but our TLS engine is unavailable
# 4.\d+.\d+ TLS is required, but unavailable
# 4.\d+.\d+ Cannot start TLS: handshake failure

Example 2: censor the per-recipient delivery status text so that it does
not reveal the destination command or filename when a remote sender re-
quests confirmation of successful delivery.

/etc/postfix/main.cf:
local_delivery_status_filter = pcre:/etc/postfix/local_dsn_filter

/etc/postfix/local_dsn_filter:
/^(2\S+ delivered to file).+/ $1
/^(2\S+ delivered to command).+/ $1

Notes:

• This feature will NOT override the soft_bounce safety net.

• This feature will change the enhanced status code and text that
is logged to the maillog file, and that is reported to the sender
in delivery confirmation or non-delivery notifications.

This feature is available in Postfix 3.0 and later.

default_destination_concurrency_failed_cohort_limit (default: 1)
How many pseudo-cohorts must suffer connection or handshake failure be-
fore a specific destination is considered unavailable (and further de-
livery is suspended). Specify zero to disable this feature. A destina-
tion's pseudo-cohort failure count is reset each time a delivery com-
pletes without connection or handshake failure for that specific desti-
nation.

A pseudo-cohort is the number of deliveries equal to a destination's de-
livery concurrency.

Use transport_destination_concurrency_failed_cohort_limit to specify a
transport-specific override, where transport is the master.cf name of
the message delivery transport.

This feature is available in Postfix 2.5. The default setting is compat-
ible with earlier Postfix versions.

default_destination_concurrency_limit (default: 20)
The default maximal number of parallel deliveries to the same destina-
tion. This is the default limit for delivery via the lmtp(8), pipe(8),
smtp(8) and virtual(8) delivery agents. With a per-destination recipi-
ent limit > 1, a destination is a domain, otherwise it is a recipient.

Use transport_destination_concurrency_limit to specify a transport-spe-
cific override, where transport is the master.cf name of the message de-
livery transport.

default_destination_concurrency_negative_feedback (default: 1)
The per-destination amount of delivery concurrency negative feedback,
after a delivery completes with a connection or handshake failure. Feed-
back values are in the range 0..1 inclusive. With negative feedback,
concurrency is decremented at the beginning of a sequence of length
1/feedback. This is unlike positive feedback, where concurrency is in-
cremented at the end of a sequence of length 1/feedback.

As of Postfix version 2.5, negative feedback cannot reduce delivery con-
currency to zero. Instead, a destination is marked dead (further deliv-
ery suspended) after the failed pseudo-cohort count reaches $de-
fault_destination_concurrency_failed_cohort_limit (or $transport_desti-
nation_concurrency_failed_cohort_limit). To make the scheduler com-
pletely immune to connection or handshake failures, specify a zero feed-
back value and a zero failed pseudo-cohort limit.

Specify one of the following forms:

number

number / number
Constant feedback. The value must be in the range 0..1 inclusive.
The default setting of "1" is compatible with Postfix versions
before 2.5, where a destination's delivery concurrency is throt-
tled down to zero (and further delivery suspended) after a single
failed pseudo-cohort.

number / concurrency
Variable feedback of "number / (delivery concurrency)". The num-
ber must be in the range 0..1 inclusive. With number equal to
"1", a destination's delivery concurrency is decremented by 1 af-
ter each failed pseudo-cohort.

A pseudo-cohort is the number of deliveries equal to a destination's de-
livery concurrency.

Use transport_destination_concurrency_negative_feedback to specify a
transport-specific override, where transport is the master.cf name of
the message delivery transport.

This feature is available in Postfix 2.5. The default setting is compat-
ible with earlier Postfix versions.

default_destination_concurrency_positive_feedback (default: 1)
The per-destination amount of delivery concurrency positive feedback,
after a delivery completes without connection or handshake failure.
Feedback values are in the range 0..1 inclusive. The concurrency in-
creases until it reaches the per-destination maximal concurrency limit.
With positive feedback, concurrency is incremented at the end of a se-
quence with length 1/feedback. This is unlike negative feedback, where
concurrency is decremented at the start of a sequence of length 1/feed-
back.

Specify one of the following forms:

number

number / number
Constant feedback. The value must be in the range 0..1 inclu-
sive. The default setting of "1" is compatible with Postfix ver-
sions before 2.5, where a destination's delivery concurrency dou-
bles after each successful pseudo-cohort.

number / concurrency
Variable feedback of "number / (delivery concurrency)". The num-
ber must be in the range 0..1 inclusive. With number equal to
"1", a destination's delivery concurrency is incremented by 1 af-
ter each successful pseudo-cohort.

A pseudo-cohort is the number of deliveries equal to a destination's de-
livery concurrency.

Use transport_destination_concurrency_positive_feedback to specify a
transport-specific override, where transport is the master.cf name of
the message delivery transport.

This feature is available in Postfix 2.5 and later.

default_destination_rate_delay (default: 0s)
The default amount of delay that is inserted between individual message
deliveries to the same destination and over the same message delivery
transport. Specify a non-zero value to rate-limit those message deliver-
ies to at most one per $default_destination_rate_delay.

The resulting behavior depends on the value of the corresponding
per-destination recipient limit.

• With a corresponding per-destination recipient limit > 1, the
rate delay specifies the time between deliveries to the same do-
main. Different domains are delivered in parallel, subject to
the process limits specified in master.cf.

• With a corresponding per-destination recipient limit equal to 1,
the rate delay specifies the time between deliveries to the same
recipient. Different recipients are delivered in parallel, sub-
ject to the process limits specified in master.cf.

To enable the delay, specify a non-zero time value (an integral value
plus an optional one-letter suffix that specifies the time unit).

Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
The default time unit is s (seconds).

NOTE: the delay is enforced by the queue manager. The delay timer state
does not survive "postfix reload" or "postfix stop".

Use transport_destination_rate_delay to specify a transport-specific
override, where transport is the master.cf name of the message delivery
transport.

NOTE: with a non-zero _destination_rate_delay, specify a transport_des-
tination_concurrency_failed_cohort_limit of 10 or more to prevent Post-
fix from deferring all mail for the same destination after only one con-
nection or handshake error.

This feature is available in Postfix 2.5 and later.

default_destination_recipient_limit (default: 50)
The default maximal number of recipients per message delivery. This is
the default limit for delivery via the lmtp(8), pipe(8), smtp(8) and
virtual(8) delivery agents.

Setting this parameter to a value of 1 affects email deliveries as fol-
lows:

• It changes the meaning of the corresponding per-destination con-
currency limit, from concurrency of deliveries to the same domain
into concurrency of deliveries to the same recipient. Different
recipients are delivered in parallel, subject to the process lim-
its specified in master.cf.

• It changes the meaning of the corresponding per-destination rate
delay, from the delay between deliveries to the same domain into
the delay between deliveries to the same recipient. Again, dif-
ferent recipients are delivered in parallel, subject to the
process limits specified in master.cf.

• It changes the meaning of other corresponding per-destination
settings in a similar manner, from settings for delivery to the
same domain into settings for delivery to the same recipient.

Use transport_destination_recipient_limit to specify a transport-spe-
cific override, where transport is the master.cf name of the message de-
livery transport.

default_extra_recipient_limit (default: 1000)
The default value for the extra per-transport limit imposed on the num-
ber of in-memory recipients. This extra recipient space is reserved for
the cases when the Postfix queue manager's scheduler preempts one mes-
sage with another and suddenly needs some extra recipient slots for the
chosen message in order to avoid performance degradation.

Use transport_extra_recipient_limit to specify a transport-specific
override, where transport is the master.cf name of the message delivery
transport.

default_filter_nexthop (default: empty)
When a content_filter or FILTER request specifies no explicit next-hop
destination, use $default_filter_nexthop instead; when that value is
empty, use the domain in the recipient address. Specify "default_fil-
ter_nexthop = $myhostname" for compatibility with Postfix version 2.6
and earlier, or specify an explicit next-hop destination with each con-
tent_filter value or FILTER action.

This feature is available in Postfix 2.7 and later.

default_minimum_delivery_slots (default: 3)
How many recipients a message must have in order to invoke the Postfix
queue manager's scheduling algorithm at all. Messages which would never
accumulate at least this many delivery slots (subject to slot cost para-
meter as well) are never preempted.

Use transport_minimum_delivery_slots to specify a transport-specific
override, where transport is the master.cf name of the message delivery
transport.

default_privs (default: nobody)
The default rights used by the local(8) delivery agent for delivery to
an external file or command. These rights are used when delivery is re-
quested from an aliases(5) file that is owned by root, or when delivery
is done on behalf of root. DO NOT SPECIFY A PRIVILEGED USER OR THE POST-
FIX OWNER.

default_process_limit (default: 100)
The default maximal number of Postfix child processes that provide a
given service. This limit can be overruled for specific services in the
master.cf file.

default_rbl_reply (default: see postconf -d output)
The default Postfix SMTP server response template for a request that is
rejected by an RBL-based restriction. This template can be overruled by
specific entries in the optional rbl_reply_maps lookup table.

This feature is available in Postfix 2.0 and later.

The template does not support Postfix configuration parameter $name sub-
stitution. Instead, it supports exactly one level of $name substitution
for the following attributes:

$client
The client hostname and IP address, formatted as name[address].

$client_address
The client IP address.

$client_name
The client hostname or "unknown". See reject_unknown_client_host-
name for more details.

$reverse_client_name
The client hostname from address->name lookup, or "unknown". See
reject_unknown_reverse_client_hostname for more details.

$helo_name
The hostname given in HELO or EHLO command or empty string.

$rbl_class
The denylisted entity type: Client host, Helo command, Sender ad-
dress, or Recipient address.

$rbl_code
The numerical SMTP response code, as specified with the
maps_rbl_reject_code configuration parameter. Note: The numerical
SMTP response code is required, and must appear at the start of
the reply. With Postfix version 2.3 and later this information
may be followed by an RFC 3463 enhanced status code.

$rbl_domain
The RBL domain (without any =address-filter) where $rbl_what is
denylisted.

$rbl_reason
The reason why $rbl_what is denylisted, or an empty string.

$rbl_what
The entity that is denylisted (an IP address, a hostname, a do-
main name, or an email address whose domain was denylisted).

$recipient
The recipient address or <> in case of the null address.

$recipient_domain
The recipient domain or empty string.

$recipient_name
The recipient address localpart or <> in case of null address.

$sender
The sender address or <> in case of the null address.

$sender_domain
The sender domain or empty string.

$sender_name
The sender address localpart or <> in case of the null address.

${name?value}

${name?{value}} (Postfix >= 3.0)
Expands to value when $name is non-empty.

${name:value}

${name:{value}} (Postfix >= 3.0)
Expands to value when $name is empty.

${name?{value1}:{value2}} (Postfix >= 3.0)
Expands to value1 when $name is non-empty, value2 otherwise.

Instead of $name you can also specify ${name} or $(name).

Note: when an enhanced status code is specified in an RBL reply tem-
plate, it is subject to modification. The following transformations are
needed when the same RBL response template is used for client, helo,
sender, or recipient access restrictions.

• When rejecting a sender address, the Postfix SMTP server will
transform a recipient DSN status (e.g., 4.1.1-4.1.6) into the
corresponding sender DSN status, and vice versa.

• When rejecting non-address information (such as the HELO command
argument or the client hostname/address), the Postfix SMTP server
will transform a sender or recipient DSN status into a generic
non-address DSN status (e.g., 4.0.0).

default_recipient_limit (default: 20000)
The default per-transport upper limit on the number of in-memory recipi-
ents. These limits take priority over the global qmgr_message_recipi-
ent_limit after the message has been assigned to the respective trans-
ports. See also default_extra_recipient_limit and qmgr_message_recipi-
ent_minimum.

Use transport_recipient_limit to specify a transport-specific override,
where transport is the master.cf name of the message delivery transport.

default_recipient_refill_delay (default: 5s)
The default per-transport maximum delay between refilling recipients.
When not all message recipients fit into memory at once, keep loading
more of them at least once every this many seconds. This is used to
make sure the recipients are refilled in a timely manner even when $de-
fault_recipient_refill_limit is too high for too slow deliveries.

Use transport_recipient_refill_delay to specify a transport-specific
override, where transport is the master.cf name of the message delivery
transport.

This feature is available in Postfix 2.4 and later.

default_recipient_refill_limit (default: 100)
The default per-transport limit on the number of recipients refilled at
once. When not all message recipients fit into memory at once, keep
loading more of them in batches of at least this many at a time. See
also $default_recipient_refill_delay, which may result in recipient
batches lower than this when this limit is too high for too slow deliv-
eries.

Use transport_recipient_refill_limit to specify a transport-specific
override, where transport is the master.cf name of the message delivery
transport.

This feature is available in Postfix 2.4 and later.

default_transport (default: smtp)
The default mail delivery transport and next-hop destination for the de-
fault domain class: recipient domains that do not match $mydestination,
$inet_interfaces, $proxy_interfaces, $virtual_alias_domains, $vir-
tual_mailbox_domains, or $relay_domains. This information will not be
used when sender_dependent_default_transport_maps returns a result, and
may be overridden with the transport(5) table.

For recipient domains in the default domain class:

• In order of decreasing precedence, the delivery transport is
taken from 1) $transport_maps, 2) $sender_dependent_de-
fault_transport_maps or $default_transport.

• In order of decreasing precedence, the nexthop destination is
taken from 1) $transport_maps, 2) $sender_dependent_de-
fault_transport_maps or $default_transport, 3) $sender_depen-
dent_relayhost_maps or $relayhost or the recipient domain.

Specify a string of the form transport:nexthop, where transport is the
name of a mail delivery transport defined in master.cf. The :nexthop
destination is optional; its syntax is documented in the manual page of
the corresponding delivery agent. In the case of SMTP or LMTP, specify
one or more destinations separated by comma or whitespace (with Postfix
3.5 and later).

Example:

default_transport = uucp:relayhostname

default_transport_rate_delay (default: 0s)
The default amount of delay that is inserted between individual message
deliveries over the same message delivery transport, regardless of des-
tination. Specify a non-zero value to rate-limit those message deliver-
ies to at most one per $default_transport_rate_delay.

Use transport_transport_rate_delay to specify a transport-specific over-
ride, where the initial transport is the master.cf name of the message
delivery transport.

Example: throttle outbound SMTP mail to at most 3 deliveries per minute.

/etc/postfix/main.cf:
smtp_transport_rate_delay = 20s

To enable the delay, specify a non-zero time value (an integral value
plus an optional one-letter suffix that specifies the time unit).

Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
The default time unit is s (seconds).

NOTE: the delay is enforced by the queue manager.

This feature is available in Postfix 3.1 and later.

default_verp_delimiters (default: +=)
The two default VERP delimiter characters. These are used when no ex-
plicit delimiters are specified with the SMTP XVERP command or with the
"sendmail -XV" command-line option (Postfix 2.2 and earlier: -V). Spec-
ify characters that are allowed by the verp_delimiter_filter setting.

This feature is available in Postfix 1.1 and later.

defer_code (default: 450)
The numerical Postfix SMTP server response code when a remote SMTP
client request is rejected by the "defer" restriction.

Do not change this unless you have a complete understanding of RFC 5321.

defer_service_name (default: defer)
The name of the defer service. This service is implemented by the
bounce(8) daemon and maintains a record of failed delivery attempts and
generates non-delivery notifications.

This feature is available in Postfix 2.0 and later.

defer_transports (default: empty)
The names of message delivery transports that should not deliver mail
unless someone issues "sendmail -q" or equivalent. Specify zero or more
mail delivery transport names that appear in the first field of mas-
ter.cf.

Example:

defer_transports = smtp

delay_logging_resolution_limit (default: 2)
The maximal number of digits after the decimal point when logging delay
values. Specify a number in the range 0..6.

To improve readability, delays are logged with only a limited number of
significant digits:

• Delays >= 100s are rounded off to an integral number of seconds.

• Delays < 10^-$delay_logging_resolution_limit are rounded off to
"0".

• Other delays are rounded off to two significant digits, or fewer
if the result would exceed the delay_logging_resolution_limit.

The format of the "delays=a/b/c/d" logging is as follows:

• a = Time from message arrival to last active queue entry.

• b = Time from last active queue entry to the beginning of connec-
tion setup.

• c = Time in connection setup. With SMTP, that is the time before
sending the MAIL FROM command: with a new connection, that in-
cludes DNS lookups, and protocol handshakes with TCP, EHLO, and
STARTTLS; with a reused connection, that includes DNS lookups,
connection cache lookup by domain or IP address, and a liveness
probe with RSET.

• d = Time in message transmission. With SMTP, that starts with
sending MAIL FROM.

This feature is available in Postfix 2.3 and later.

delay_notice_recipient (default: postmaster)
The recipient of postmaster notifications with the message headers of
mail that cannot be delivered within $delay_warning_time time units.

See also: delay_warning_time, notify_classes.

delay_warning_time (default: 0h)
The time after which the sender receives a copy of the message headers
of mail that is still queued. The confirm_delay_cleared parameter con-
trols sender notification when the delay clears up.

To enable this feature, specify a non-zero time value (an integral value
plus an optional one-letter suffix that specifies the time unit).

Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
The default time unit is h (hours).

See also: delay_notice_recipient, notify_classes, confirm_delay_cleared.

deliver_lock_attempts (default: 20)
The maximal number of attempts to acquire an exclusive lock on a mailbox
file or bounce(8) logfile.

deliver_lock_delay (default: 1s)
The time between attempts to acquire an exclusive lock on a mailbox file
or bounce(8) logfile.

destination_concurrency_feedback_debug (default: no)
Make the queue manager's feedback algorithm verbose for performance
analysis purposes.

This feature is available in Postfix 2.5 and later.

detect_8bit_encoding_header (default: yes)
Automatically detect 8BITMIME body content by looking at Content-Trans-
fer-Encoding: message headers; historically, this behavior was
hard-coded to be "always on".

This feature is available in Postfix 2.5 and later.

disable_dns_lookups (default: no)
Disable DNS lookups in the Postfix SMTP and LMTP clients. When disabled,
hosts are looked up with the getaddrinfo() system library routine which
normally also looks in /etc/hosts. As of Postfix 2.11, this parameter
is deprecated; use smtp_dns_support_level instead.

DNS lookups are enabled by default.

disable_mime_input_processing (default: no)
Turn off MIME processing while receiving mail. This means that no spe-
cial treatment is given to Content-Type: message headers, and that all
text after the initial message headers is considered to be part of the
message body.

This feature is available in Postfix 2.0 and later.

Mime input processing is enabled by default, and is needed in order to
recognize MIME headers in message content.

disable_mime_output_conversion (default: no)
Disable the conversion of 8BITMIME format to 7BIT format. Mime output
conversion is needed when the destination does not advertise 8BITMIME
support.

This feature is available in Postfix 2.0 and later.

disable_verp_bounces (default: no)
Disable sending one bounce report per recipient.

The default, one per recipient, is what ezmlm needs.

This feature is available in Postfix 1.1 and later.

disable_vrfy_command (default: no)
Disable the SMTP VRFY command. This stops some techniques used to har-
vest email addresses.

Example:

disable_vrfy_command = no

dns_ncache_ttl_fix_enable (default: no)
Enable a workaround for future libc incompatibility. The Postfix imple-
mentation of RFC 2308 negative reply caching relies on the promise that
res_query() and res_search() invoke res_send(), which returns the server
response in an application buffer even if the requested record does not
exist. If this promise is broken, specify "yes" to enable a workaround
for DNS reputation lookups.

This feature is available in Postfix 3.1 and later.

dnsblog_reply_delay (default: 0s)
A debugging aid to artificially delay DNS responses.

This feature is available in Postfix 2.8.

dnsblog_service_name (default: dnsblog)
The name of the dnsblog(8) service entry in master.cf. This service per-
forms DNS allow/denylist lookups.

This feature is available in Postfix 2.8 and later.

dnssec_probe (default: ns:.)
The DNS query type (default: "ns") and DNS query name (default: ".")
that Postfix may use to determine whether DNSSEC validation is avail-
able.

Background: DNSSEC validation is needed for Postfix DANE support; this
ensures that Postfix receives TLSA records with secure TLS server cer-
tificate info. When DNSSEC validation is unavailable, mail deliveries
using opportunistic DANE will not be protected by server certificate
info in TLSA records, and mail deliveries using mandatory DANE will not
be made at all.

By default, a Postfix process will send a DNSSEC probe after 1) the
process made a DNS query that requested DNSSEC validation, 2) the
process did not receive a DNSSEC validated response to this query or to
an earlier query, and 3) the process did not already send a DNSSEC
probe.

When the DNSSEC probe has no response, or when the response is not
DNSSEC validated, Postfix logs a warning that DNSSEC validation may be
unavailable.

Example:

warning: DNSSEC validation may be unavailable
warning: reason: dnssec_probe 'ns:.' received a response that is not DNSSEC validated
warning: reason: dnssec_probe 'ns:.' received no response: Server failure

Possible reasons why DNSSEC validation may be unavailable:

• The local /etc/resolv.conf file specifies a DNS resolver that
does not validate DNSSEC signatures (that's $queue_direc-
tory/etc/resolv.conf when a Postfix daemon runs in a chroot
jail).

• The local system library does not pass on the "DNSSEC validated"
bit to Postfix, or Postfix does not know how to ask the library
to do that.

By default, the DNSSEC probe asks for the DNS root zone NS records, be-
cause resolvers should always have that information cached. If Postfix
runs on a network where the DNS root zone is not reachable, specify a
different probe, or specify an empty dnssec_probe value to disable the
feature.

This feature is available in Postfix 3.6 and later. It was backported to
Postfix versions 3.5.9, 3.4.19, 3.3.16. 3.2.21.

dont_remove (default: 0)
Don't remove queue files and save them to the "saved" mail queue. This
is a debugging aid. To inspect the envelope information and content of
a Postfix queue file, use the postcat(1) command.

double_bounce_sender (default: double-bounce)
The sender address of postmaster notifications that are generated by the
mail system. All mail to this address is silently discarded, in order to
terminate mail bounce loops.

duplicate_filter_limit (default: 1000)
The maximal number of addresses remembered by the address duplicate fil-
ter for aliases(5) or virtual(5) alias expansion, or for showq(8) queue
displays.

empty_address_default_transport_maps_lookup_key (default: <>)
The sender_dependent_default_transport_maps search string that will be
used instead of the null sender address.

This feature is available in Postfix 2.7 and later.

empty_address_local_login_sender_maps_lookup_key (default: <>)
The lookup key to be used in local_login_sender_maps tables, instead of
the null sender address.

This feature is available in Postfix 3.6 and later.

empty_address_recipient (default: MAILER-DAEMON)
The recipient of mail addressed to the null address. Postfix does not
accept such addresses in SMTP commands, but they may still be created
locally as the result of configuration or software error.

empty_address_relayhost_maps_lookup_key (default: <>)
The sender_dependent_relayhost_maps search string that will be used in-
stead of the null sender address.

This feature is available in Postfix 2.5 and later. With earlier ver-
sions, sender_dependent_relayhost_maps lookups were skipped for the null
sender address.

enable_errors_to (default: no)
Report mail delivery errors to the address specified with the non-stan-
dard Errors-To: message header, instead of the envelope sender address
(this feature is removed with Postfix version 2.2, is turned off by de-
fault with Postfix version 2.1, and is always turned on with older Post-
fix versions).

enable_idna2003_compatibility (default: no)
Enable 'transitional' compatibility between IDNA2003 and IDNA2008, when
converting UTF-8 domain names to/from the ASCII form that is used for
DNS lookups. Specify "yes" for compatibility with Postfix <= 3.1 (not
recommended). This affects the conversion of domain names that contain
for example the German sz and the Greek zeta. See https://uni-
code.org/cldr/utility/idna.jsp for more examples.

This feature is available in Postfix 3.2 and later.

enable_long_queue_ids (default: no)
Enable long, non-repeating, queue IDs (queue file names). The benefit
of non-repeating names is simpler logfile analysis and easier queue mi-
gration (there is no need to run "postsuper" to change queue file names
that don't match their message file inode number).

Note: see below for how to convert long queue file names to Postfix <=
2.8.

Changing the parameter value to "yes" has the following effects:

• Existing queue file names are not affected.

• New queue files are created with names such as 3Pt2mN2VXxznjll.
These are encoded in a 52-character alphabet that contains digits
(0-9), upper-case letters (B-Z) and lower-case letters (b-z). For
safety reasons the vowels (AEIOUaeiou) are excluded from the al-
phabet. The name format is: 6 or more characters for the time in
seconds, 4 characters for the time in microseconds, the 'z'; the
remainder is the file inode number encoded in the first 51 char-
acters of the 52-character alphabet.

• New messages have a Message-ID header with queueID@myhostname.

• The mailq (postqueue -p) output has a wider Queue ID column. The
number of whitespace-separated fields is not changed.

• The hash_queue_depth algorithm uses the first characters of the
queue file creation time in microseconds, after conversion into
hexadecimal representation. This produces the same queue hashing
behavior as if the queue file name was created with "en-
able_long_queue_ids = no".

Changing the parameter value to "no" has the following effects:

• Existing long queue file names are renamed to the short form
(while running "postfix reload" or "postsuper").

• New queue files are created with names such as C3CD21F3E90 from a
hexadecimal alphabet that contains digits (0-9) and upper-case
letters (A-F). The name format is: 5 characters for the time in
microseconds; the remainder is the file inode number.

• New messages have a Message-ID header with YYYYMMDDHH-
MMSS.queueid@myhostname, where YYYYMMDDHHMMSS are the year,
month, day, hour, minute and second.

• The mailq (postqueue -p) output has the same format as with Post-
fix <= 2.8.

• The hash_queue_depth algorithm uses the first characters of the
queue file name, with the hexadecimal representation of the file
creation time in microseconds.

Before migration to Postfix <= 2.8, the following commands are required
to convert long queue file names into short names:

# postfix stop
# postconf enable_long_queue_ids=no
# postsuper

Repeat the postsuper command until it reports no more queue file name
changes.

This feature is available in Postfix 2.9 and later.

enable_original_recipient (default: yes)
Enable support for the original recipient address after an address is
rewritten to a different address (for example with aliasing or with
canonical mapping).

The original recipient address is used as follows:

Final delivery
With "enable_original_recipient = yes", the original recipient
address is stored in the X-Original-To message header. This
header may be used to distinguish between different recipients
that share the same mailbox.

Recipient deduplication
With "enable_original_recipient = yes", the cleanup(8) daemon
performs duplicate recipient elimination based on the content of
(original recipient, maybe-rewritten recipient) pairs. Other-
wise, the cleanup(8) daemon performs duplicate recipient elimina-
tion based only on the maybe-rewritten recipient address.

Note: with Postfix <= 3.2 the "setting enable_original_recipient = no"
breaks address verification for addresses that are aliased or otherwise
rewritten (Postfix is unable to store the address verification result
under the original probe destination address; instead, it can store the
result only under the rewritten address).

This feature is available in Postfix 2.1 and later. Postfix version 2.0
behaves as if this parameter is always set to yes. Postfix versions be-
fore 2.0 have no support for the original recipient address.

enable_threaded_bounces (default: no)
Enable non-delivery, success, and delay notifications that link to the
original message by including a References: and In-Reply-To: header with
the original Message-ID value. There are advantages and disadvantages to
consider.

advantage
This allows mail readers to present a delivery status notifica-
tion in the same email thread as the original message.

disadvantage
This makes it easy for users to mistakenly delete the whole email
thread (all related messages), instead of deleting only the
non-delivery notification.

This feature is available in Postfix 3.6 and later.

error_notice_recipient (default: postmaster)
The recipient of postmaster notifications about mail delivery problems
that are caused by policy, resource, software or protocol errors. These
notifications are enabled with the notify_classes parameter.

error_service_name (default: error)
The name of the error(8) pseudo delivery agent. This service always re-
turns mail as undeliverable.

This feature is available in Postfix 2.0 and later.

execution_directory_expansion_filter (default: see postconf -d output)
Restrict the characters that the local(8) delivery agent allows in $name
expansions of $command_execution_directory. Characters outside the al-
lowed set are replaced by underscores.

This feature is available in Postfix 2.2 and later.

expand_owner_alias (default: no)
When delivering to an alias "aliasname" that has an "owner-aliasname"
companion alias, set the envelope sender address to the expansion of the
"owner-aliasname" alias. Normally, Postfix sets the envelope sender ad-
dress to the name of the "owner-aliasname" alias.

export_environment (default: see postconf -d output)
The list of environment variables that a Postfix process will export to
non-Postfix processes. The TZ variable is needed for sane time keeping
on System-V-ish systems.

Specify a list of names and/or name=value pairs, separated by whitespace
or comma. Specify "{ name=value }" to protect whitespace or comma in pa-
rameter values (whitespace after the opening "{" and before the closing
"}" is ignored). The form name=value is supported with Postfix version
2.1 and later; the use of {} is supported with Postfix 3.0 and later.

Example:

export_environment = TZ PATH=/bin:/usr/bin

extract_recipient_limit (default: 10240)
The maximal number of recipient addresses that Postfix will extract from
message headers when mail is submitted with "sendmail -t".

This feature was removed in Postfix version 2.1.

fallback_relay (default: empty)
Optional list of relay hosts for SMTP destinations that can't be found
or that are unreachable. With Postfix 2.3 this parameter is renamed to
smtp_fallback_relay.

By default, mail is returned to the sender when a destination is not
found, and delivery is deferred when a destination is unreachable.

The fallback relays must be SMTP destinations. Specify a domain, host,
host:port, [host]:port, [address] or [address]:port; the form [host]
turns off MX lookups. If you specify multiple SMTP destinations, Post-
fix will try them in the specified order.

Note: before Postfix 2.2, do not use the fallback_relay feature when re-
laying mail for a backup or primary MX domain. Mail would loop between
the Postfix MX host and the fallback_relay host when the final destina-
tion is unavailable.

• In main.cf specify "relay_transport = relay",

• In master.cf specify "-o fallback_relay =" (i.e., empty) at the
end of the relay entry.

• In transport maps, specify "relay:nexthop..." as the right-hand
side for backup or primary MX domain entries.

Postfix version 2.2 and later will not use the fallback_relay feature
for destinations that it is MX host for.

fallback_transport (default: empty)
Optional message delivery transport that the local(8) delivery agent
should use for names that are not found in the aliases(5) or UNIX pass-
word database.

The precedence of local(8) delivery features from high to low is:
aliases, .forward files, mailbox_transport_maps, mailbox_transport,
mailbox_command_maps, mailbox_command, home_mailbox, mail_spool_direc-
tory, fallback_transport_maps, fallback_transport and luser_relay.

fallback_transport_maps (default: empty)
Optional lookup tables with per-recipient message delivery transports
for recipients that the local(8) delivery agent could not find in the
aliases(5) or UNIX password database.

For safety reasons, this feature does not allow $number substitutions in
regular expression maps.

This feature is available in Postfix 2.3 and later.

fast_flush_domains (default: $relay_domains)
Optional list of destinations that are eligible for per-destination log-
files with mail that is queued to those destinations.

By default, Postfix maintains "fast flush" logfiles only for destina-
tions that the Postfix SMTP server is willing to relay to (i.e. the de-
fault is: "fast_flush_domains = $relay_domains"; see the relay_domains
parameter in the postconf(5) manual).

Specify a list of hosts or domains, "/file/name" patterns or "type:ta-
ble" lookup tables, separated by commas and/or whitespace. Continue
long lines by starting the next line with whitespace. A "/file/name"
pattern is replaced by its contents; a "type:table" lookup table is
matched when the domain or its parent domain appears as lookup key.

Pattern matching of domain names is controlled by the presence or ab-
sence of "fast_flush_domains" in the parent_domain_matches_subdomains
parameter value.

Specify "fast_flush_domains =" (i.e., empty) to disable the feature al-
together.

fast_flush_purge_time (default: 7d)
The time after which an empty per-destination "fast flush" logfile is
deleted.

You can specify the time as a number, or as a number followed by a let-
ter that indicates the time unit: s=seconds, m=minutes, h=hours, d=days,
w=weeks. The default time unit is days.

fast_flush_refresh_time (default: 12h)
The time after which a non-empty but unread per-destination "fast flush"
logfile needs to be refreshed. The contents of a logfile are refreshed
by requesting delivery of all messages listed in the logfile.

You can specify the time as a number, or as a number followed by a let-
ter that indicates the time unit: s=seconds, m=minutes, h=hours, d=days,
w=weeks. The default time unit is hours.

fault_injection_code (default: 0)
Force specific internal tests to fail, to test the handling of errors
that are difficult to reproduce otherwise.

flush_service_name (default: flush)
The name of the flush(8) service. This service maintains per-destination
logfiles with the queue file names of mail that is queued for those des-
tinations.

This feature is available in Postfix 2.0 and later.

force_mime_input_conversion (default: no)
Convert body content that claims to be 8-bit into quoted-printable, be-
fore header_checks, body_checks, Milters, and before after-queue content
filters. This feature does not affect messages that are sent into
smtpd_proxy_filter.

The typical use case is an MTA that applies this conversion before sign-
ing outbound messages, so that the signatures will remain valid when a
message is later delivered to an MTA that does not announce 8BITMIME
support, or when a message line exceeds the SMTP length limit.

This feature is available in Postfix >= 3.9.

fork_attempts (default: 5)
The maximal number of attempts to fork() a child process.

fork_delay (default: 1s)
The delay between attempts to fork() a child process.

forward_expansion_filter (default: see postconf -d output)
Restrict the characters that the local(8) delivery agent allows in $name
expansions of $forward_path. Characters outside the allowed set are re-
placed by underscores.

forward_path (default: see postconf -d output)
The local(8) delivery agent search list for finding a .forward file with
user-specified delivery methods. The first file that is found is used.

The forward_path value is not subject to Postfix configuration parameter
$name expansion. Instead, the following $name expansions are done on
forward_path before the search actually happens. The result of $name
expansion is filtered with the character set that is specified with the
forward_expansion_filter parameter.

$user The recipient's username.

$shell The recipient's login shell pathname.

$home The recipient's home directory.

$recipient
The full recipient address.

$extension
The optional recipient address extension.

$domain
The recipient domain.

$local The entire recipient localpart.

$recipient_delimiter
The address extension delimiter that was found in the recipient
address (Postfix 2.11 and later), or the 'first' delimiter speci-
fied with the system-wide recipient address extension delimiter
(Postfix 3.5.22, 3.6.12, 3.7.8, 3.8.3 and later). Historically,
this was always the system-wide recipient address extension de-
limiter (Postfix 2.10 and earlier).

${name?value}

${name?{value}} (Postfix >= 3.0)
Expands to value when $name is non-empty.

${name:value}

${name:{value}} (Postfix >= 3.0)
Expands to value when $name is empty.

${name?{value1}:{value2}} (Postfix >= 3.0)
Expands to value1 when $name is non-empty, value2 otherwise.

Instead of $name you can also specify ${name} or $(name).

Examples:

forward_path = /var/forward/$user
forward_path =
/var/forward/$user/.forward$recipient_delimiter$extension,
/var/forward/$user/.forward

frozen_delivered_to (default: yes)
Update the local(8) delivery agent's idea of the Delivered-To: address
(see prepend_delivered_header) only once, at the start of a delivery at-
tempt; do not update the Delivered-To: address while expanding aliases
or .forward files.

This feature is available in Postfix 2.3 and later. With older Postfix
releases, the behavior is as if this parameter is set to "no". The old
setting can be expensive with deeply nested aliases or .forward files.
When an alias or .forward file changes the Delivered-To: address, it
ties up one queue file and one cleanup process instance while mail is
being forwarded.

full_name_encoding_charset (default: utf-8)
The character set name (also called "charset") that Postfix will output
when it automatically generates an RFC 2047 encoded full name. Encoding
non-ASCII full names can avoid the need to use SMTPUTF8, and therefore
can avoid incompatibility with sites that do not support SMTPUTF8.

The encoded names look like "=?charset?q?gibberish?=" with quoted-print-
able encoding, or "=?charset?b?gibberish?=" with base64 encoding. Post-
fix uses quoted-printable encoding for a full name that is short or
mostly printable ASCII, and uses base64 otherwise.

Background: when a message without a From: header is submitted with the
Postfix sendmail(1) command, the Postfix cleanup(8) daemon will add a
From: header and will try to use the sender's full name specified with
the Postfix sendmail(1) "-F" option, with the Postfix sendmail(1) "NAME"
environment variable, or with the GECOS field in the UNIX password data-
base. In the latter case, Postfix will replace the "&" character with
the login name, with a lowercase ASCII first character converted to up-
percase.

NOTE: Postfix does not convert between character sets. The full_name_en-
coding_charset parameter specifies the character set of the full name in
the Postfix sendmail "-F" option, in the Postfix sendmail "NAME" envi-
ronment variable, or in the GECOS field of the UNIX password database.
The parameter value is also part of the encoded full name, and informs a
Mail User Agent how to display the decoded gibberish.

Specify a valid character set name such as "utf-8" or "iso-8859-1 (spec-
ify the latter for full names that use the Latin1 encoding). The char-
acter set name is case insensitive. When a character set name violates
RFC 2047 syntax, Postfix will log a warning and will skip the full name.

This feature is available in Postfix >= 3.10.

hash_queue_depth (default: 1)
The number of subdirectory levels for queue directories listed with the
hash_queue_names parameter. Queue hashing is implemented by creating one
or more levels of directories with one-character names. Originally,
these directory names were equal to the first characters of the queue
file name, with the hexadecimal representation of the file creation time
in microseconds.

With long queue file names, queue hashing produces the same results as
with short names. The file creation time in microseconds is converted
into hexadecimal form before the result is used for queue hashing. The
base 16 encoding gives finer control over the number of subdirectories
than is possible with the base 52 encoding of long queue file names.

After changing the hash_queue_names or hash_queue_depth parameter, exe-
cute the command "postfix reload".

hash_queue_names (default: deferred, defer)
The names of queue directories that are split across multiple subdirec-
tory levels.

Before Postfix version 2.2, the default list of hashed queues was sig-
nificantly larger. Claims about improvements in file system technology
suggest that hashing of the incoming and active queues is no longer
needed. Fewer hashed directories speed up the time needed to restart
Postfix.

After changing the hash_queue_names or hash_queue_depth parameter, exe-
cute the command "postfix reload".

header_address_token_limit (default: 10240)
The maximal number of address tokens are allowed in an address message
header. Information that exceeds the limit is discarded. The limit is
enforced by the cleanup(8) server.

header_checks (default: empty)
Optional lookup tables for content inspection of primary non-MIME mes-
sage headers, as specified in the header_checks(5) manual page.

header_from_format (default: standard)
The format of the Postfix-generated From: header. This setting affects
the appearance of 'full name' information when a local program such as
/bin/mail submits a message without a From: header through the Postfix
sendmail(1) command.

Specify one of the following:

standard (default)
Produce a header formatted as "From: name <address>". This is
the default as of Postfix 3.3.

obsolete
Produce a header formatted as "From: address (name)". This is the
behavior prior to Postfix 3.3.

Notes:

• Postfix generates the format "From: address" when name informa-
tion is unavailable or the envelope sender address is empty. This
is the same behavior as prior to Postfix 3.3.

• In the standard form, the name will be quoted if it contains spe-
cials as defined in RFC 5322, or the "!%" address operators.

• The Postfix sendmail(1) command gets name information from the -F
command-line option, from the NAME environment variable, or from
the UNIX password file.

This feature is available in Postfix 3.3 and later.

header_size_limit (default: 102400)
The maximal amount of memory in bytes for storing a message header. If
a header is larger, the excess is discarded. The limit is enforced by
the cleanup(8) server.

helpful_warnings (default: yes)
Log warnings about problematic configuration settings, and provide help-
ful suggestions.

This feature is available in Postfix 2.0 and later.

home_mailbox (default: empty)
Optional pathname of a mailbox file relative to a local(8) user's home
directory.

Specify a pathname ending in "/" for qmail-style delivery.

Examples:

home_mailbox = Mailbox
home_mailbox = Maildir/

hopcount_limit (default: 50)
The maximal number of Received: message headers that is allowed in the
primary message headers. A message that exceeds the limit is bounced, in
order to stop a mailer loop.

html_directory (default: see postconf -d output)
The location of Postfix HTML files that describe how to build, configure
or operate a specific Postfix subsystem or feature.

ignore_mx_lookup_error (default: no)
Ignore DNS MX lookups that produce no response. By default, the Postfix
SMTP client defers delivery and tries again after some delay. This be-
havior is required by the SMTP standard.

Specify "ignore_mx_lookup_error = yes" to force a DNS A record lookup
instead. This violates the SMTP standard and can result in mis-delivery
of mail.

ignore_srv_lookup_error (default: no)
When SRV record lookup fails, fall back to MX or IP address lookup as if
SRV record lookup was not enabled.

This feature is available in Postfix 3.8 and later.

import_environment (default: see postconf -d output)
The list of environment variables that a privileged Postfix process will
import from a non-Postfix parent process, or name=value environment
overrides. Unprivileged utilities will enforce the name=value over-
rides, but otherwise will not change their process environment. Exam-
ples of relevant environment variables:

TZ May be needed for sane time keeping on most System-V-ish systems.

DISPLAY
Needed for debugging Postfix daemons with an X-windows debugger.

XAUTHORITY
Needed for debugging Postfix daemons with an X-windows debugger.

MAIL_CONFIG
Needed to make "postfix -c" work.

POSTLOG_SERVICE
Needed to make "maillog_file" work during daemon process initial-
ization.

POSTLOG_HOSTNAME
Needed to make "maillog_file" work during daemon process initial-
ization.

Specify a list of names and/or name=value pairs, separated by whitespace
or comma. Specify "{ name=value }" to protect whitespace or comma in en-
vironment variable values (whitespace after the opening "{" and before
the closing "}" is ignored). The form name=value is supported with Post-
fix version 2.1 and later; the use of {} is supported with Postfix 3.0
and later.

in_flow_delay (default: 1s)
Time to pause before accepting a new message, when the message arrival
rate exceeds the message delivery rate. This feature is turned on by de-
fault (it's disabled on SCO UNIX due to an SCO bug).

With the default 100 Postfix SMTP server process limit, "in_flow_delay =
1s" limits the mail inflow to 100 messages per second above the number
of messages delivered per second.

Specify 0 to disable the feature. Valid delays are 0..10.

inet_interfaces (default: all)
The local network interface addresses that this mail system receives
mail on. Specify "all" to receive mail on all network interfaces (de-
fault), "loopback-only" to receive mail on loopback network interfaces
only (Postfix version 2.2 and later), or zero or more IPv4 or IPv6 ad-
dresses (IPv6 is supported in Postfix version 2.2 and later). The para-
meter also controls whether Postfix will accept mail for user@[ip.ad-
dress], and prevents Postfix from delivering mail to a host that has
equal or larger MX preference. Specify an empty value if Postfix does
not receive mail over the network, or if all network listeners have an
explicit IP address in master.cf.

Note 1: you need to stop and start Postfix when this parameter changes.

Note 2: address information may be enclosed inside [], but this form is
not required here.

When smtp_bind_address and/or smtp_bind_address6 are not specified, the
inet_interfaces setting may constrain the source IP address for an out-
bound SMTP or LMTP connection as described below.

The following text is specific to SMTP and IPv4. The same reasoning ap-
plies to the IPv6 protocol, and to the Postfix LMTP client. To disable
IPv4 or IPv6 support in the Postfix SMTP and LMTP client, use inet_pro-
tocols.

• When inet_interfaces specifies one IPv4 address, and that is not
a loopback address, the Postfix SMTP client uses that as the
source address for outbound IPv4 connections.

• Otherwise, the Postfix SMTP client does not constrain the source
IPv4 address, and connects using a system-chosen source IPv4 ad-
dress. This includes the cases where inet_interfaces is empty,
where it specifies all, or where it contains no IPv4 address, one
IPv4 address that is a loopback address, or multiple IPv4 ad-
dresses.

A Postfix SMTP client may fail to reach some remote SMTP servers when
the client source IP address is constrained explicitly with
smtp_bind_address or smtp_bind_address6, or implicitly with inet_inter-
faces. This can happen when Postfix runs on a multi-homed system such as
a firewall, the Postfix SMTP source client IP address is constrained to
one specific network interface, and the remote SMTP server must be
reached through a different interface. Setting smtp_bind_address to
0.0.0.0 avoids the potential problem for IPv4, and setting smtp_bind_ad-
dress6 to :: solves the problem for IPv6.

A better solution for multi-homed systems is to leave inet_interfaces at
the default value and instead use explicit IP addresses in the master.cf
SMTP server definitions. This preserves the Postfix SMTP client's loop
detection, by ensuring that each side of the firewall knows that the
other IP address is still the same host. Setting $inet_interfaces to a
single IPv4 and/or IPV6 address is primarily useful with virtual hosting
of domains on secondary IP addresses, when each IP address serves a dif-
ferent domain (and has a different $myhostname setting).

See also the proxy_interfaces parameter, for network addresses that are
forwarded to Postfix by way of a proxy or address translator.

Examples:

inet_interfaces = all (DEFAULT)
inet_interfaces = loopback-only (Postfix version 2.2 and later)
inet_interfaces = 127.0.0.1
inet_interfaces = 127.0.0.1, [::1] (Postfix version 2.2 and later)
inet_interfaces = 192.168.1.2, 127.0.0.1

inet_protocols (default: see 'postconf -d' output)
The Internet protocols Postfix will attempt to use when making or ac-
cepting connections. Specify one or more of "ipv4" or "ipv6", separated
by whitespace or commas. The form "all" is equivalent to "ipv4, ipv6" or
"ipv4", depending on whether the operating system implements IPv6.

With Postfix 2.8 and earlier the default is "ipv4". For backwards com-
patibility with these releases, the Postfix 2.9 and later upgrade proce-
dure appends an explicit "inet_protocols = ipv4" setting to main.cf when
no explicit setting is present. This compatibility workaround will be
phased out as IPv6 deployment becomes more common.

This feature is available in Postfix 2.2 and later.

Note: you MUST stop and start Postfix after changing this parameter.

On systems that pre-date IPV6_V6ONLY support (RFC 3493), an IPv6 server
will also accept IPv4 connections, even when IPv4 is turned off with the
inet_protocols parameter. On systems with IPV6_V6ONLY support, Postfix
will use separate server sockets for IPv6 and IPv4, and each will accept
only connections for the corresponding protocol.

When IPv4 support is enabled via the inet_protocols parameter, Postfix
will look up DNS type A records, and will convert IPv4-in-IPv6 client IP
addresses (::ffff:1.2.3.4) to their original IPv4 form (1.2.3.4). The
latter is needed on hosts that pre-date IPV6_V6ONLY support (RFC 3493).

When IPv6 support is enabled via the inet_protocols parameter, Postfix
will do DNS type AAAA record lookups.

When both IPv4 and IPv6 support are enabled, the Postfix SMTP client
will choose the protocol as specified with the smtp_address_preference
parameter. Postfix versions before 2.8 attempt to connect via IPv6 be-
fore attempting to use IPv4.

Examples:

inet_protocols = ipv4
inet_protocols = all (DEFAULT)
inet_protocols = ipv6
inet_protocols = ipv4, ipv6

info_log_address_format (default: external)
The email address form that will be used in non-debug logging (info,
warning, etc.). As of Postfix 3.5 when an address localpart contains
spaces or other special characters, the localpart will be quoted, for
example:

from=<"name with spaces"@example.com>

Older Postfix versions would log the internal (unquoted) form:

from=<name with spaces@example.com>

The external and internal forms are identical for the vast majority of
email addresses that contain no spaces or other special characters in
the localpart.

The logging in external form is consistent with the address form that
Postfix 3.2 and later prefer for most table lookups. This is therefore
the more useful form for non-debug logging.

Specify "info_log_address_format = internal" for backwards compatibil-
ity.

Postfix uses the unquoted form internally, because an attacker can spec-
ify an email address in different forms by playing games with quotes and
backslashes. An attacker should not be able to use such games to circum-
vent Postfix access policies.

This feature is available in Postfix 3.5 and later.

initial_destination_concurrency (default: 5)
The initial per-destination concurrency level for parallel delivery to
the same destination. With per-destination recipient limit > 1, a des-
tination is a domain, otherwise it is a recipient.

Use transport_initial_destination_concurrency to specify a trans-
port-specific override, where transport is the master.cf name of the
message delivery transport (Postfix 2.5 and later).

Warning: with concurrency of 1, one bad message can be enough to block
all mail to a site.

internal_mail_filter_classes (default: empty)
What categories of Postfix-generated mail are subject to before-queue
content inspection by non_smtpd_milters, header_checks and body_checks.
Specify zero or more of the following, separated by whitespace or comma.

bounce Inspect the content of delivery status notifications.

notify Inspect the content of postmaster notifications by the smtp(8)
and smtpd(8) processes.

NOTE: It's generally not safe to enable content inspection of Post-
fix-generated email messages. The user is warned.

This feature is available in Postfix 2.3 and later.

invalid_hostname_reject_code (default: 501)
The numerical Postfix SMTP server response code when the client HELO or
EHLO command parameter is rejected by the reject_invalid_helo_hostname
restriction.

Do not change this unless you have a complete understanding of RFC 5321.

ipc_idle (default: version dependent)
The time after which a client closes an idle internal communication
channel. The purpose is to allow Postfix daemon processes to terminate
voluntarily after they become idle. This is used, for example, by the
Postfix address resolving and rewriting clients.

With Postfix 2.4 the default value was reduced from 100s to 5s.

ipc_timeout (default: 3600s)
The time limit for sending or receiving information over an internal
communication channel. The purpose is to break out of deadlock situa-
tions. If the time limit is exceeded the software aborts with a fatal
error.

ipc_ttl (default: 1000s)
The time after which a client closes an active internal communication
channel. The purpose is to allow Postfix daemon processes to terminate
voluntarily after reaching their client limit. This is used, for exam-
ple, by the Postfix address resolving and rewriting clients.

This feature is available in Postfix 2.1 and later.

known_tcp_ports (default: lmtp=24, smtp=25, smtps=submissions=465, submis-
sion=587)
Optional setting that avoids lookups in the services(5) database. This
feature was implemented to address inconsistencies in the name of the
port "465" service. The ABNF is:

known_tcp_ports = empty | name-to-port *("," name-to-port)
name-to-port = 1*(service-name "=') port-number

The comma is required. Whitespace is optional but it cannot appear in-
side a service name or port number.

This feature is available in Postfix 3.6 and later.

line_length_limit (default: 2048)
Upon input, long lines are chopped up into pieces of at most this
length; upon delivery, long lines are reconstructed.

lmdb_map_size (default: 16777216)
The initial OpenLDAP LMDB database size limit in bytes. Each time a
database becomes full, its size limit is doubled.

This feature is available in Postfix 2.11 and later.

lmtp_address_preference (default: ipv6)
The LMTP-specific version of the smtp_address_preference configuration
parameter. See there for details.

This feature is available in Postfix 2.8 and later.

lmtp_address_verify_target (default: rcpt)
The LMTP-specific version of the smtp_address_verify_target configura-
tion parameter. See there for details.

This feature is available in Postfix 3.0 and later.

lmtp_assume_final (default: no)
When a remote LMTP server announces no DSN support, assume that the
server performs final delivery, and send "delivered" delivery status no-
tifications instead of "relayed". The default setting is backwards com-
patible to avoid the infinitesimal possibility of breaking existing
LMTP-based content filters.

lmtp_balance_inet_protocols (default: yes)
The LMTP-specific version of the smtp_balance_inet_protocols configura-
tion parameter. See there for details.

This feature is available in Postfix 3.3 and later.

lmtp_bind_address (default: empty)
The LMTP-specific version of the smtp_bind_address configuration parame-
ter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_bind_address6 (default: empty)
The LMTP-specific version of the smtp_bind_address6 configuration para-
meter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_bind_address_enforce (default: empty)
The LMTP-specific version of the smtp_bind_address_enforce configuration
parameter. See there for details.

This feature is available in Postfix 3.7 and later.

lmtp_body_checks (default: empty)
The LMTP-specific version of the smtp_body_checks configuration parame-
ter. See there for details.

This feature is available in Postfix 2.5 and later.

lmtp_cache_connection (default: yes)
Keep Postfix LMTP client connections open for up to $max_idle seconds.
When the LMTP client receives a request for the same connection the con-
nection is reused.

This parameter is available in Postfix version 2.2 and earlier. With
Postfix version 2.3 and later, see lmtp_connection_cache_on_demand,
lmtp_connection_cache_destinations, or lmtp_connection_reuse_time_limit.

The effectiveness of cached connections will be determined by the number
of remote LMTP servers in use, and the concurrency limit specified for
the Postfix LMTP client. Cached connections are closed under any of the
following conditions:

• The Postfix LMTP client idle time limit is reached. This limit
is specified with the Postfix max_idle configuration parameter.

• A delivery request specifies a different destination than the one
currently cached.

• The per-process limit on the number of delivery requests is
reached. This limit is specified with the Postfix max_use con-
figuration parameter.

• Upon the onset of another delivery request, the remote LMTP
server associated with the current session does not respond to
the RSET command.

Most of these limitations have been with the Postfix connection cache
that is shared among multiple LMTP client programs.

lmtp_cname_overrides_servername (default: yes)
The LMTP-specific version of the smtp_cname_overrides_servername config-
uration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_connect_timeout (default: 0s)
The Postfix LMTP client time limit for completing a TCP connection, or
zero (use the operating system built-in time limit). When no connection
can be made within the deadline, the LMTP client tries the next address
on the mail exchanger list.

Example:

lmtp_connect_timeout = 30s

lmtp_connection_cache_destinations (default: empty)
The LMTP-specific version of the smtp_connection_cache_destinations con-
figuration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_connection_cache_on_demand (default: yes)
The LMTP-specific version of the smtp_connection_cache_on_demand config-
uration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_connection_cache_time_limit (default: 2s)
The LMTP-specific version of the smtp_connection_cache_time_limit con-
figuration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_connection_reuse_count_limit (default: 0)
The LMTP-specific version of the smtp_connection_reuse_count_limit con-
figuration parameter. See there for details.

This feature is available in Postfix 2.11 and later.

lmtp_connection_reuse_time_limit (default: 300s)
The LMTP-specific version of the smtp_connection_reuse_time_limit con-
figuration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_data_done_timeout (default: 600s)
The Postfix LMTP client time limit for sending the LMTP ".", and for re-
ceiving the remote LMTP server response. When no response is received
within the deadline, a warning is logged that the mail may be delivered
multiple times.

lmtp_data_init_timeout (default: 120s)
The Postfix LMTP client time limit for sending the LMTP DATA command,
and for receiving the remote LMTP server response.

lmtp_data_xfer_timeout (default: 180s)
The Postfix LMTP client time limit for sending the LMTP message content.
When the connection stalls for more than $lmtp_data_xfer_timeout the
LMTP client terminates the transfer.

lmtp_defer_if_no_mx_address_found (default: no)
The LMTP-specific version of the smtp_defer_if_no_mx_address_found con-
figuration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_delivery_status_filter (default: empty)
The LMTP-specific version of the smtp_delivery_status_filter configura-
tion parameter. See there for details.

This feature is available in Postfix 3.0 and later.

lmtp_destination_concurrency_limit (default: $default_destination_concur-
rency_limit)
The maximal number of parallel deliveries to the same destination via
the lmtp message delivery transport. This limit is enforced by the queue
manager. The message delivery transport name is the first field in the
entry in the master.cf file.

lmtp_destination_recipient_limit (default: $default_destination_recipi-
ent_limit)
The maximal number of recipients per message for the lmtp message deliv-
ery transport. This limit is enforced by the queue manager. The message
delivery transport name is the first field in the entry in the master.cf
file.

Setting this parameter to a value of 1 changes the meaning of lmtp_des-
tination_concurrency_limit from concurrency per domain into concurrency
per recipient.

lmtp_discard_lhlo_keyword_address_maps (default: empty)
Lookup tables, indexed by the remote LMTP server address, with case in-
sensitive lists of LHLO keywords (pipelining, starttls, auth, etc.) that
the Postfix LMTP client will ignore in the LHLO response from a remote
LMTP server. See lmtp_discard_lhlo_keywords for details. The table is
not indexed by hostname for consistency with smtpd_discard_ehlo_key-
word_address_maps.

This feature is available in Postfix 2.3 and later.

lmtp_discard_lhlo_keywords (default: empty)
A case insensitive list of LHLO keywords (pipelining, starttls, auth,
etc.) that the Postfix LMTP client will ignore in the LHLO response from
a remote LMTP server.

This feature is available in Postfix 2.3 and later.

Notes:

• Specify the silent-discard pseudo keyword to prevent this action
from being logged.

• Use the lmtp_discard_lhlo_keyword_address_maps feature to discard
LHLO keywords selectively.

lmtp_dns_reply_filter (default: empty)
Optional filter for Postfix LMTP client DNS lookup results. See
smtp_dns_reply_filter for details including an example.

This feature is available in Postfix 3.0 and later.

lmtp_dns_resolver_options (default: empty)
The LMTP-specific version of the smtp_dns_resolver_options configuration
parameter. See there for details.

This feature is available in Postfix 2.8 and later.

lmtp_dns_support_level (default: empty)
The LMTP-specific version of the smtp_dns_support_level configuration
parameter. See there for details.

This feature is available in Postfix 2.11 and later.

lmtp_enforce_tls (default: no)
The LMTP-specific version of the smtp_enforce_tls configuration parame-
ter. See there for details.

This feature is deprecated as of Postfix 3.9. Specify lmtp_tls_secu-
rity_level instead.

This feature is available in Postfix 2.3 and later.

lmtp_fallback_relay (default: empty)
Optional list of relay hosts for LMTP destinations that can't be found
or that are unreachable. In main.cf elements are separated by white-
space or commas.

By default, mail is returned to the sender when a destination is not
found, and delivery is deferred when a destination is unreachable.

The fallback relays must be TCP destinations, specified without a lead-
ing "inet:" prefix. Specify a host or host:port. Since MX lookups do
not apply with LMTP, there is no need to use the "[host]" or
"[host]:port" forms. If you specify multiple LMTP destinations, Postfix
will try them in the specified order.

This feature is available in Postfix 3.1 and later.

lmtp_generic_maps (default: empty)
The LMTP-specific version of the smtp_generic_maps configuration parame-
ter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_header_checks (default: empty)
The LMTP-specific version of the smtp_header_checks configuration para-
meter. See there for details.

This feature is available in Postfix 2.5 and later.

lmtp_host_lookup (default: dns)
The LMTP-specific version of the smtp_host_lookup configuration parame-
ter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_lhlo_name (default: $myhostname)
The hostname to send in the LMTP LHLO command.

The default value is the machine hostname. Specify a hostname or
[ip.add.re.ss] or [ip:v6:add:re::ss].

This information can be specified in the main.cf file for all LMTP
clients, or it can be specified in the master.cf file for a specific
client, for example:

/etc/postfix/master.cf:
mylmtp ... lmtp -o lmtp_lhlo_name=foo.bar.com

This feature is available in Postfix 2.3 and later.

lmtp_lhlo_timeout (default: 300s)
The Postfix LMTP client time limit for sending the LHLO command, and for
receiving the initial remote LMTP server response.

Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
The default time unit is s (seconds).

lmtp_line_length_limit (default: 998)
The LMTP-specific version of the smtp_line_length_limit configuration
parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_mail_timeout (default: 300s)
The Postfix LMTP client time limit for sending the MAIL FROM command,
and for receiving the remote LMTP server response.

lmtp_mime_header_checks (default: empty)
The LMTP-specific version of the smtp_mime_header_checks configuration
parameter. See there for details.

This feature is available in Postfix 2.5 and later.

lmtp_min_data_rate (default: 500)
The LMTP-specific version of the smtp_min_data_rate configuration para-
meter. See there for details.

This feature is available in Postfix 3.7 and later.

lmtp_mx_address_limit (default: 5)
The LMTP-specific version of the smtp_mx_address_limit configuration pa-
rameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_mx_session_limit (default: 2)
The LMTP-specific version of the smtp_mx_session_limit configuration pa-
rameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_nested_header_checks (default: empty)
The LMTP-specific version of the smtp_nested_header_checks configuration
parameter. See there for details.

This feature is available in Postfix 2.5 and later.

lmtp_per_record_deadline (default: no)
The LMTP-specific version of the smtp_per_record_deadline configuration
parameter. See there for details.

This feature is available in Postfix 2.9 and later.

lmtp_per_request_deadline (default: no)
The LMTP-specific version of the smtp_per_request_deadline configuration
parameter. See there for details.

This feature is available in Postfix 3.7 and later.

lmtp_pix_workaround_delay_time (default: 10s)
The LMTP-specific version of the smtp_pix_workaround_delay_time configu-
ration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_pix_workaround_maps (default: empty)
The LMTP-specific version of the smtp_pix_workaround_maps configuration
parameter. See there for details.

This feature is available in Postfix 2.4 and later.

lmtp_pix_workaround_threshold_time (default: 500s)
The LMTP-specific version of the smtp_pix_workaround_threshold_time con-
figuration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_pix_workarounds (default: empty)
The LMTP-specific version of the smtp_pix_workaround configuration para-
meter. See there for details.

This feature is available in Postfix 2.4 and later.

lmtp_quit_timeout (default: 300s)
The Postfix LMTP client time limit for sending the QUIT command, and for
receiving the remote LMTP server response.

lmtp_quote_rfc821_envelope (default: yes)
The LMTP-specific version of the smtp_quote_rfc821_envelope configura-
tion parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_randomize_addresses (default: yes)
The LMTP-specific version of the smtp_randomize_addresses configuration
parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_rcpt_timeout (default: 300s)
The Postfix LMTP client time limit for sending the RCPT TO command, and
for receiving the remote LMTP server response.

lmtp_reply_filter (default: empty)
The LMTP-specific version of the smtp_reply_filter configuration parame-
ter. See there for details.

This feature is available in Postfix 2.7 and later.

lmtp_rset_timeout (default: 20s)
The Postfix LMTP client time limit for sending the RSET command, and for
receiving the remote LMTP server response. The LMTP client sends RSET in
order to finish a recipient address probe, or to verify that a cached
connection is still alive.

lmtp_sasl_auth_cache_name (default: empty)
The LMTP-specific version of the smtp_sasl_auth_cache_name configuration
parameter. See there for details.

This feature is available in Postfix 2.5 and later.

lmtp_sasl_auth_cache_time (default: 90d)
The LMTP-specific version of the smtp_sasl_auth_cache_time configuration
parameter. See there for details.

This feature is available in Postfix 2.5 and later.

lmtp_sasl_auth_enable (default: no)
Enable SASL authentication in the Postfix LMTP client.

lmtp_sasl_auth_soft_bounce (default: yes)
The LMTP-specific version of the smtp_sasl_auth_soft_bounce configura-
tion parameter. See there for details.

This feature is available in Postfix 2.5 and later.

lmtp_sasl_mechanism_filter (default: empty)
The LMTP-specific version of the smtp_sasl_mechanism_filter configura-
tion parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_sasl_password_maps (default: empty)
Optional Postfix LMTP client lookup tables with one username:password
entry per host or domain. If a remote host or domain has no user-
name:password entry, then the Postfix LMTP client will not attempt to
authenticate to the remote host.

lmtp_sasl_password_result_delimiter (default: :)
The LMTP-specific version of the smtp_sasl_password_result_delimiter
configuration parameter. See there for details.

lmtp_sasl_path (default: empty)
Implementation-specific information that is passed through to the SASL
plug-in implementation that is selected with lmtp_sasl_type. Typically
this specifies the name of a configuration file or rendezvous point.

This feature is available in Postfix 2.3 and later.

lmtp_sasl_security_options (default: noplaintext, noanonymous)
SASL security options; as of Postfix 2.3 the list of available features
depends on the SASL client implementation that is selected with
lmtp_sasl_type.

The following security features are defined for the cyrus client SASL
implementation:

noplaintext
Disallow authentication methods that use plaintext passwords.

noactive
Disallow authentication methods that are vulnerable to non-dic-
tionary active attacks.

nodictionary
Disallow authentication methods that are vulnerable to passive
dictionary attacks.

noanonymous
Disallow anonymous logins.

Example:

lmtp_sasl_security_options = noplaintext

lmtp_sasl_tls_security_options (default: $lmtp_sasl_security_options)
The LMTP-specific version of the smtp_sasl_tls_security_options configu-
ration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_sasl_tls_verified_security_options (default: $lmtp_sasl_tls_security_op-
tions)
The LMTP-specific version of the smtp_sasl_tls_verified_security_options
configuration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_sasl_type (default: cyrus)
The SASL plug-in type that the Postfix LMTP client should use for au-
thentication. The available types are listed with the "postconf -A"
command.

This feature is available in Postfix 2.3 and later.

lmtp_send_dummy_mail_auth (default: no)
The LMTP-specific version of the smtp_send_dummy_mail_auth configuration
parameter. See there for details.

This feature is available in Postfix 2.9 and later.

lmtp_send_xforward_command (default: no)
Send an XFORWARD command to the remote LMTP server when the LMTP LHLO
server response announces XFORWARD support. This allows an lmtp(8) de-
livery agent, used for content filter message injection, to forward the
name, address, protocol and HELO name of the original client to the con-
tent filter and downstream LMTP server. Before you change the value to
yes, it is best to make sure that your content filter supports this com-
mand.

This feature is available in Postfix 2.1 and later.

lmtp_sender_dependent_authentication (default: no)
The LMTP-specific version of the smtp_sender_dependent_authentication
configuration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_skip_5xx_greeting (default: yes)
The LMTP-specific version of the smtp_skip_5xx_greeting configuration
parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_skip_quit_response (default: no)
Wait for the response to the LMTP QUIT command.

lmtp_starttls_timeout (default: 300s)
The LMTP-specific version of the smtp_starttls_timeout configuration pa-
rameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tcp_port (default: 24)
The default TCP port that the Postfix LMTP client connects to. Specify
a symbolic name (see services(5)) or a numeric port.

lmtp_tls_CAfile (default: empty)
The LMTP-specific version of the smtp_tls_CAfile configuration parame-
ter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_CApath (default: empty)
The LMTP-specific version of the smtp_tls_CApath configuration parame-
ter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_block_early_mail_reply (default: empty)
The LMTP-specific version of the smtp_tls_block_early_mail_reply config-
uration parameter. See there for details.

This feature is available in Postfix 2.7 and later.

lmtp_tls_cert_file (default: empty)
The LMTP-specific version of the smtp_tls_cert_file configuration para-
meter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_chain_files (default: empty)
The LMTP-specific version of the smtp_tls_chain_files configuration pa-
rameter. See there for details.

This feature is available in Postfix 3.4 and later.

lmtp_tls_ciphers (default: medium)
The LMTP-specific version of the smtp_tls_ciphers configuration parame-
ter. See there for details.

This feature is available in Postfix 2.6 and later.

lmtp_tls_connection_reuse (default: no)
The LMTP-specific version of the smtp_tls_connection_reuse configuration
parameter. See there for details.

This feature is available in Postfix 3.4 and later.

lmtp_tls_dcert_file (default: empty)
The LMTP-specific version of the smtp_tls_dcert_file configuration para-
meter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_dkey_file (default: $lmtp_tls_dcert_file)
The LMTP-specific version of the smtp_tls_dkey_file configuration para-
meter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_eccert_file (default: empty)
The LMTP-specific version of the smtp_tls_eccert_file configuration pa-
rameter. See there for details.

This feature is available in Postfix 2.6 and later, when Postfix is com-
piled and linked with OpenSSL 1.0.0 or later.

lmtp_tls_eckey_file (default: empty)
The LMTP-specific version of the smtp_tls_eckey_file configuration para-
meter. See there for details.

This feature is available in Postfix 2.6 and later, when Postfix is com-
piled and linked with OpenSSL 1.0.0 or later.

lmtp_tls_enable_rpk (default: yes)
The LMTP-specific version of the smtp_tls_enable_rpk configuration para-
meter. See there for details.

This feature is available in Postfix 3.9 and later.

lmtp_tls_enforce_peername (default: yes)
The LMTP-specific version of the smtp_tls_enforce_peername configuration
parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_exclude_ciphers (default: empty)
The LMTP-specific version of the smtp_tls_exclude_ciphers configuration
parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_fingerprint_cert_match (default: empty)
The LMTP-specific version of the smtp_tls_fingerprint_cert_match config-
uration parameter. See there for details.

This feature is available in Postfix 2.5 and later.

lmtp_tls_fingerprint_digest (default: see postconf -d output)
The LMTP-specific version of the smtp_tls_fingerprint_digest configura-
tion parameter. See there for details.

This feature is available in Postfix 2.5 and later.

lmtp_tls_force_insecure_host_tlsa_lookup (default: no)
The LMTP-specific version of the smtp_tls_force_inse-
cure_host_tlsa_lookup configuration parameter. See there for details.

This feature is available in Postfix 2.11 and later.

lmtp_tls_key_file (default: $lmtp_tls_cert_file)
The LMTP-specific version of the smtp_tls_key_file configuration parame-
ter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_loglevel (default: 0)
The LMTP-specific version of the smtp_tls_loglevel configuration parame-
ter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_mandatory_ciphers (default: medium)
The LMTP-specific version of the smtp_tls_mandatory_ciphers configura-
tion parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_mandatory_exclude_ciphers (default: empty)
The LMTP-specific version of the smtp_tls_mandatory_exclude_ciphers con-
figuration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_mandatory_protocols (default: see 'postconf -d' output)
The LMTP-specific version of the smtp_tls_mandatory_protocols configura-
tion parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_note_starttls_offer (default: no)
The LMTP-specific version of the smtp_tls_note_starttls_offer configura-
tion parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_per_site (default: empty)
The LMTP-specific version of the smtp_tls_per_site configuration parame-
ter. See there for details.

This feature is deprecated as of Postfix 3.9. Specify lmtp_tls_pol-
icy_maps instead.

This feature is available in Postfix 2.3 and later.

lmtp_tls_policy_maps (default: empty)
The LMTP-specific version of the smtp_tls_policy_maps configuration pa-
rameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_protocols (default: see 'postconf -d' output)
The LMTP-specific version of the smtp_tls_protocols configuration para-
meter. See there for details.

This feature is available in Postfix 2.6 and later.

lmtp_tls_scert_verifydepth (default: 9)
The LMTP-specific version of the smtp_tls_scert_verifydepth configura-
tion parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_secure_cert_match (default: nexthop)
The LMTP-specific version of the smtp_tls_secure_cert_match configura-
tion parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_security_level (default: empty)
The LMTP-specific version of the smtp_tls_security_level configuration
parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_servername (default: empty)
The LMTP-specific version of the smtp_tls_servername configuration para-
meter. See there for details.

This feature is available in Postfix 3.4 and later.

lmtp_tls_session_cache_database (default: empty)
The LMTP-specific version of the smtp_tls_session_cache_database config-
uration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_session_cache_timeout (default: 3600s)
The LMTP-specific version of the smtp_tls_session_cache_timeout configu-
ration parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_trust_anchor_file (default: empty)
The LMTP-specific version of the smtp_tls_trust_anchor_file configura-
tion parameter. See there for details.

This feature is available in Postfix 2.11 and later.

lmtp_tls_verify_cert_match (default: hostname)
The LMTP-specific version of the smtp_tls_verify_cert_match configura-
tion parameter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_wrappermode (default: no)
The LMTP-specific version of the smtp_tls_wrappermode configuration pa-
rameter. See there for details.

This feature is available in Postfix 3.0 and later.

lmtp_use_tls (default: no)
The LMTP-specific version of the smtp_use_tls configuration parameter.
See there for details.

This feature is deprecated as of Postfix 3.9. Specify lmtp_tls_secu-
rity_level instead.

This feature is available in Postfix 2.3 and later.

lmtp_xforward_timeout (default: 300s)
The Postfix LMTP client time limit for sending the XFORWARD command, and
for receiving the remote LMTP server response.

In case of problems the client does NOT try the next address on the mail
exchanger list.

This feature is available in Postfix 2.1 and later.

local_command_shell (default: empty)
Optional shell program for local(8) delivery to non-Postfix commands.
By default, non-Postfix commands are executed directly; commands are
given to the default shell (typically, /bin/sh) only when they contain
shell meta characters or shell built-in commands.

"sendmail's restricted shell" (smrsh) is what most people will use in
order to restrict what programs can be run from e.g. .forward files
(smrsh is part of the Sendmail distribution).

Note: when a shell program is specified, it is invoked even when the
command contains no shell built-in commands or meta characters.

Example:

local_command_shell = /some/where/smrsh -c
local_command_shell = /bin/bash -c

local_delivery_status_filter (default: $default_delivery_status_filter)
Optional filter for the local(8) delivery agent to change the status
code or explanatory text of successful or unsuccessful deliveries. See
default_delivery_status_filter for details.

This feature is available in Postfix 3.0 and later.

local_destination_concurrency_limit (default: 2)
The maximal number of parallel deliveries via the local mail delivery
transport to the same recipient (when "local_destination_recipient_limit
= 1") or the maximal number of parallel deliveries to the same local do-
main (when "local_destination_recipient_limit > 1"). This limit is en-
forced by the queue manager. The message delivery transport name is the
first field in the entry in the master.cf file.

A low limit of 2 is recommended, just in case someone has an expensive
shell command in a .forward file or in an alias (e.g., a mailing list
manager). You don't want to run lots of those at the same time.

local_destination_recipient_limit (default: 1)
The maximal number of recipients per message delivery via the local mail
delivery transport. This limit is enforced by the queue manager. The
message delivery transport name is the first field in the entry in the
master.cf file.

Setting this parameter to a value > 1 changes the meaning of local_des-
tination_concurrency_limit from concurrency per recipient into concur-
rency per domain.

local_header_rewrite_clients (default: permit_inet_interfaces)
Rewrite or add message headers in mail from these clients, updating in-
complete addresses with the domain name in $myorigin or $mydomain, and
adding missing headers.

See the append_at_myorigin and append_dot_mydomain parameters for de-
tails of how domain names are appended to incomplete addresses.

See remote_header_rewrite_domain to optionally rewrite or add message
headers in mail from other clients.

Specify a list of zero or more of the following:

permit_inet_interfaces
Append the domain name in $myorigin or $mydomain when the client
IP address matches $inet_interfaces. This is enabled by default.

permit_mynetworks
Append the domain name in $myorigin or $mydomain when the client
IP address matches any network or network address listed in
$mynetworks. This setting will not prevent remote mail header ad-
dress rewriting when mail from a remote client is forwarded by a
neighboring system.

permit_sasl_authenticated
Append the domain name in $myorigin or $mydomain when the client
is successfully authenticated via the RFC 4954 (AUTH) protocol.

permit_tls_clientcerts
Append the domain name in $myorigin or $mydomain when the remote
SMTP client TLS certificate fingerprint or public key fingerprint
(Postfix 2.9 and later) is listed in $relay_clientcerts. The
fingerprint digest algorithm is configurable via the
smtpd_tls_fingerprint_digest parameter (hard-coded as md5 prior
to Postfix version 2.5).
The default algorithm is sha256 with Postfix >= 3.6 and the com-
patibility_level set to 3.6 or higher. With Postfix <= 3.5, the
default algorithm is md5. The best-practice algorithm is now
sha256. Recent advances in hash function cryptanalysis have led
to md5 and sha1 being deprecated in favor of sha256. However, as
long as there are no known "second pre-image" attacks against the
older algorithms, their use in this context, though not recom-
mended, is still likely safe.

permit_tls_all_clientcerts
Append the domain name in $myorigin or $mydomain when the remote
SMTP client TLS certificate is successfully verified, regardless
of whether it is listed on the server, and regardless of the cer-
tifying authority.

check_address_map type:table

type:table
Append the domain name in $myorigin or $mydomain when the client
IP address matches the specified lookup table. The lookup result
is ignored, and no subnet lookup is done. This is suitable for,
e.g., pop-before-smtp lookup tables.

Examples:

The Postfix < 2.2 backwards compatible setting: always rewrite message
headers, and always append my own domain to incomplete header addresses.

local_header_rewrite_clients = static:all

The purist (and default) setting: rewrite headers only in mail from
Postfix sendmail and in SMTP mail from this machine.

local_header_rewrite_clients = permit_inet_interfaces

The intermediate setting: rewrite header addresses and append $myorigin
or $mydomain information only with mail from Postfix sendmail, from lo-
cal clients, or from authorized SMTP clients.

Note: this setting will not prevent remote mail header address rewriting
when mail from a remote client is forwarded by a neighboring system.

local_header_rewrite_clients = permit_mynetworks,
permit_sasl_authenticated permit_tls_clientcerts
check_address_map hash:/etc/postfix/pop-before-smtp

local_login_sender_maps (default: static:*)
A list of lookup tables that are searched by the UNIX login name, and
that return a list of allowed envelope sender patterns separated by
space or comma. These sender patterns are enforced by the Postfix post-
drop(1) command. The default is backwards-compatible: every user may
specify any sender envelope address.

When no UNIX login name is available, the postdrop(1) command will
prepend "uid:" to the numerical UID and use that instead.

This feature ignores address extensions in the user-specified envelope
sender address.

Note: to enforce that the From: header address matches the envelope
sender (MAIL FROM) address, specify an external filter such as a Milter,
with the non_smtpd_milters parameter. For example:
https://github.com/magcks/milterfrom.

The following sender patterns are special; these cannot be used as part
of a longer pattern.

* This pattern allows any envelope sender address.

<> This pattern allows the empty envelope sender address. See the
empty_address_local_login_sender_maps_lookup_key configuration
parameter.

@domain
This pattern allows an envelope sender address when the '@' and
domain part match.

Examples:

/etc/postfix/main.cf:
# Allow root and postfix full control, anyone else can only
# send mail as themselves. Use "uid:" followed by the numerical
# UID when the UID has no entry in the UNIX password file.
local_login_sender_maps =
inline:{ { root = * }, { postfix = * } },
pcre:/etc/postfix/login_senders

/etc/postfix/login_senders:
# Allow both the bare username and the user@domain forms.
/(.+)/ $1 $1@example.com

This feature is available in Postfix 3.6 and later.

local_recipient_maps (default: proxy:unix:passwd.byname $alias_maps)
Lookup tables with all names or addresses of valid local recipients. A
recipient address is local when its domain matches $mydestination,
$inet_interfaces or $proxy_interfaces.

If the local_recipient_maps parameter value is non-empty (the default),
then the Postfix SMTP server queries local_recipient_maps as specified
in "Local recipient table query format", and rejects mail for unknown
local recipients. Other Postfix interfaces such as the Postfix send-
mail(1) command may still accept an "unknown" recipient.

The default local_recipient_maps setting assumes that local_transport
specifies the UNIX-compatible local(8) delivery agent which queries the
UNIX passwd database (typically, /etc.passwd) and the local aliases
database (typically, /etc/aliases). The proxy: agent allows the Postfix
SMTP server to access the UNIX passwd database from outside a chroot
jail.

For other local mail delivery configurations, see "When you need to
change the local_recipient_maps setting in main.cf".

Technically, tables listed with $local_recipient_maps are used as lists:
The Postfix SMTP server needs to know only if a lookup string is found
or not, but it does not use the result from table lookup.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

To turn off local recipient checking in the Postfix SMTP server, specify
"local_recipient_maps =" (i.e. empty).

Examples:

local_recipient_maps =

local_transport (default: local:$myhostname)
The default mail delivery transport and next-hop destination for final
delivery to domains listed with mydestination, and for [ipaddress] des-
tinations that match $inet_interfaces or $proxy_interfaces. This infor-
mation can be overruled with the transport(5) table.

By default, local mail is delivered to the transport called "local",
which is just the name of a service that is defined the master.cf file.

Beware: if you override the default local delivery agent then you need
to review the LOCAL_RECIPIENT_README document, otherwise the SMTP server
may reject mail for local recipients.

luser_relay (default: empty)
Optional catch-all destination for unknown local(8) recipients. By de-
fault, mail for unknown recipients in domains that match $mydestination,
$inet_interfaces or $proxy_interfaces is returned as undeliverable.

The luser_relay value is not subject to Postfix configuration parameter
$name expansion. Instead, the following $name expansions are done:

$domain
The recipient domain.

$extension
The recipient address extension.

$home The recipient's home directory.

$local The entire recipient address localpart.

$recipient
The full recipient address.

$shell The recipient's login shell.

$user The recipient username.

${name?value}

${name?{value}} (Postfix >= 3.0)
Expands to value when $name is non-empty.

${name:value}

${name:{value}} (Postfix >= 3.0)
Expands to value when $name is empty.

${name?{value1}:{value2}} (Postfix >= 3.0)
Expands to value1 when $name is non-empty, value2 otherwise.

Instead of $name you can also specify ${name} or $(name).

Note: luser_relay works only for the Postfix local(8) delivery agent.

Note: if you use this feature for accounts not in the UNIX password
file, then you must specify "local_recipient_maps =" (i.e. empty) in the
main.cf file, otherwise the Postfix SMTP server will reject mail for
non-UNIX accounts with "User unknown in local recipient table".

Examples:

luser_relay = $user@other.host
luser_relay = $local@other.host
luser_relay = admin+$local

mail_name (default: Postfix)
The mail system name that is displayed in Received: headers, in the SMTP
greeting banner, and in bounced mail.

mail_owner (default: postfix)
The UNIX system account that owns the Postfix queue and most Postfix
daemon processes. Specify the name of an unprivileged user account that
does not share a user or group ID with other accounts, and that owns no
other files or processes on the system. In particular, don't specify
nobody or daemon. PLEASE USE A DEDICATED USER ID AND GROUP ID.

When this parameter value is changed you need to re-run "postfix
set-permissions" (with Postfix version 2.0 and earlier: "/etc/post-
fix/post-install set-permissions".

mail_release_date (default: see postconf -d output)
The Postfix release date, in "YYYYMMDD" format.

mail_spool_directory (default: see postconf -d output)
The directory where local(8) UNIX-style mailboxes are kept. The default
setting depends on the system type. Specify a name ending in / for
maildir-style delivery.

Note: maildir delivery is done with the privileges of the recipient. If
you use the mail_spool_directory setting for maildir style delivery,
then you must create the top-level maildir directory in advance. Postfix
will not create it.

Examples:

mail_spool_directory = /var/mail
mail_spool_directory = /var/spool/mail

mail_version (default: see postconf -d output)
The version of the mail system. Stable releases are named major.mi-
nor.patchlevel. Experimental releases also include the release date. The
version string can be used in, for example, the SMTP greeting banner.

mailbox_command (default: empty)
Optional external command that the local(8) delivery agent should use
for mailbox delivery. The command is run with the user ID and the pri-
mary group ID privileges of the recipient. Exception: command delivery
for root executes with $default_privs privileges. This is not a prob-
lem, because 1) mail for root should always be aliased to a real user
and 2) don't log in as root, use "su" instead.

The following environment variables are exported to the command:

CLIENT_ADDRESS
Remote client network address. Available in Postfix version 2.2
and later.

CLIENT_HELO
Remote client EHLO command parameter. Available in Postfix ver-
sion 2.2 and later.

CLIENT_HOSTNAME
Remote client hostname. Available in Postfix version 2.2 and
later.

CLIENT_PROTOCOL
Remote client protocol. Available in Postfix version 2.2 and
later.

DOMAIN The domain part of the recipient address.

ENVID The optional RFC 3461 envelope ID. Available in Postfix version
3.9 and later

EXTENSION
The optional address extension.

HOME The recipient home directory.

LOCAL The recipient address localpart.

LOGNAME
The recipient's username.

ORIGINAL_RECIPIENT
The entire recipient address, before any address rewriting or
aliasing.

RECIPIENT
The full recipient address.

SASL_METHOD
SASL authentication method specified in the remote client AUTH
command. Available in Postfix version 2.2 and later.

SASL_SENDER
SASL sender address specified in the remote client MAIL FROM com-
mand. Available in Postfix version 2.2 and later.

SASL_USER
SASL username specified in the remote client AUTH command.
Available in Postfix version 2.2 and later.

SENDER The full sender address.

SHELL The recipient's login shell.

USER The recipient username.

Unlike other Postfix configuration parameters, the mailbox_command para-
meter is not subjected to $name substitutions. This is to make it easier
to specify shell syntax (see example below).

If you can, avoid shell meta characters because they will force Postfix
to run an expensive shell process. If you're delivering via "procmail"
then running a shell won't make a noticeable difference in the total
cost.

Note: if you use the mailbox_command feature to deliver mail sys-
tem-wide, you must set up an alias that forwards mail for root to a real
user.

Examples:

mailbox_command = /some/where/procmail
mailbox_command = /some/where/procmail -a "$EXTENSION"
mailbox_command = /some/where/maildrop -d "$USER"
-f "$SENDER" "$EXTENSION"

mailbox_command_maps (default: empty)
Optional lookup tables with per-recipient external commands to use for
local(8) mailbox delivery. Behavior is as with mailbox_command.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

mailbox_delivery_lock (default: see postconf -d output)
How to lock a UNIX-style local(8) mailbox before attempting delivery.
For a list of available file locking methods, use the "postconf -l" com-
mand.

This setting is ignored with maildir style delivery, because such deliv-
eries are safe without explicit locks.

Note: The dotlock method requires that the recipient UID or GID has
write access to the parent directory of the mailbox file.

Note: the default setting of this parameter is system dependent.

mailbox_size_limit (default: 51200000)
The maximal size of any local(8) individual mailbox or maildir file, or
zero (no limit). In fact, this limits the size of any file that is
written to upon local delivery, including files written by external com-
mands that are executed by the local(8) delivery agent. The value cannot
exceed LONG_MAX (typically, a 32-bit or 64-bit signed integer).

This limit must not be smaller than the message size limit.

mailbox_transport (default: empty)
Optional message delivery transport that the local(8) delivery agent
should use for mailbox delivery to all local recipients, whether or not
they are found in the UNIX passwd database.

mailbox_transport_maps (default: empty)
Optional lookup tables with per-recipient message delivery transports to
use for local(8) mailbox delivery, whether or not the recipients are
found in the UNIX passwd database.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

For safety reasons, this feature does not allow $number substitutions in
regular expression maps.

This feature is available in Postfix 2.3 and later.

maillog_file (default: empty)
The name of an optional logfile that is written by the Postfix post-
logd(8) service. An empty value selects logging to syslogd(8). Specify
"/dev/stdout" to select logging to standard output. Stdout logging re-
quires that Postfix is started with "postfix start-fg".

Note 1: The maillog_file parameter value must contain a prefix that is
specified with the maillog_file_prefixes parameter.

Note 2: Some Postfix non-daemon programs may still log information to
syslogd(8), before they have processed their configuration parameters
and command-line options.

This feature is available in Postfix 3.4 and later.

maillog_file_compressor (default: gzip)
The program to run after rotating $maillog_file with "postfix logro-
tate". The command is run with the rotated logfile name as its first ar-
gument.

This feature is available in Postfix 3.4 and later.

maillog_file_permissions (default: 0600)
The file access permissions that will be set when the file $maillog_file
is created for the first time, or when the file is created after an ex-
isting file is rotated. Specify one of: 0600 (only super-user read/write
access), 0640 (adds 'group' read access), or 0644 (also adds 'other'
read access). The leading '0' is optional.

This feature is available in Postfix 3.9 and later.

maillog_file_prefixes (default: /var, /dev/stdout)
A list of allowed prefixes for a maillog_file value. This is a safety
feature to contain the damage from a single configuration mistake. Spec-
ify one or more prefix strings, separated by comma or whitespace.

This feature is available in Postfix 3.4 and later.

maillog_file_rotate_suffix (default: %Y%m%d-%H%M%S)
The format of the suffix to append to $maillog_file while rotating the
file with "postfix logrotate". See strftime(3) for syntax. The default
suffix, YYYYMMDD-HHMMSS, allows logs to be rotated frequently.

This feature is available in Postfix 3.4 and later.

mailq_path (default: see postconf -d output)
Sendmail compatibility feature that specifies where the Postfix mailq(1)
command is installed. This command can be used to list the Postfix mail
queue.

manpage_directory (default: see postconf -d output)
Where the Postfix manual pages are installed.

maps_rbl_domains (default: empty)
Obsolete feature: use the reject_rbl_client feature instead.

maps_rbl_reject_code (default: 554)
The numerical Postfix SMTP server response code when a remote SMTP
client request is blocked by the reject_rbl_client, reject_rhsbl_client,
reject_rhsbl_reverse_client, reject_rhsbl_sender or reject_rhsbl_recipi-
ent restriction.

Do not change this unless you have a complete understanding of RFC 5321.

masquerade_classes (default: envelope_sender, header_sender, header_recipient)
What addresses are subject to address masquerading.

By default, address masquerading is limited to envelope sender ad-
dresses, and to header sender and header recipient addresses. This al-
lows you to use address masquerading on a mail gateway while still being
able to forward mail to users on individual machines.

Specify zero or more of: envelope_sender, envelope_recipient,
header_sender, header_recipient

masquerade_domains (default: empty)
Optional list of domains whose subdomain structure will be stripped off
in email addresses.

The list is processed left to right, and processing stops at the first
match. Thus,

masquerade_domains = foo.example.com example.com

strips "user@any.thing.foo.example.com" to "user@foo.example.com", but
strips "user@any.thing.else.example.com" to "user@example.com".

A domain name prefixed with ! means do not masquerade this domain or its
subdomains. Thus,

masquerade_domains = !foo.example.com example.com

does not change "user@any.thing.foo.example.com" or "user@foo.exam-
ple.com", but strips "user@any.thing.else.example.com" to "user@exam-
ple.com".

Note: with Postfix version 2.2, message header address masquerading hap-
pens only when message header address rewriting is enabled:

• The message is received with the Postfix sendmail(1) command,

• The message is received from a network client that matches $lo-
cal_header_rewrite_clients,

• The message is received from the network, and the re-
mote_header_rewrite_domain parameter specifies a non-empty value.

To get the behavior before Postfix version 2.2, specify "lo-
cal_header_rewrite_clients = static:all".

Example:

masquerade_domains = $mydomain

masquerade_exceptions (default: empty)
Optional list of user names that are not subjected to address masquerad-
ing, even when their addresses match $masquerade_domains.

By default, address masquerading makes no exceptions.

Examples:

masquerade_exceptions = root, mailer-daemon
masquerade_exceptions = root

master_service_disable (default: empty)
Selectively disable master(8) listener ports by service type or by ser-
vice name and type. Specify a list of service types ("inet", "unix",
"fifo", or "pass") or "name/type" tuples, where "name" is the first
field of a master.cf entry and "type" is a service type. As with other
Postfix matchlists, a search stops at the first match. Specify "!pat-
tern" to exclude a service from the list. By default, all master(8) lis-
tener ports are enabled.

Note: this feature does not support "/file/name" or "type:table" pat-
terns, nor does it support wildcards such as "*" or "all". This is in-
tentional.

Examples:

# With Postfix 2.6..2.10 use '.' instead of '/'.
# Turn on all master(8) listener ports (the default).
master_service_disable =
# Turn off only the main SMTP listener port.
master_service_disable = smtp/inet
# Turn off all TCP/IP listener ports.
master_service_disable = inet
# Turn off all TCP/IP listener ports except "foo".
master_service_disable = !foo/inet, inet

This feature is available in Postfix 2.6 and later.

max_idle (default: 100s)
The maximum amount of time that an idle Postfix daemon process waits for
an incoming connection before terminating voluntarily. This parameter
is ignored by the Postfix queue manager and by other long-lived Postfix
daemon processes.

max_use (default: 100)
The maximal number of incoming connections that a Postfix daemon process
will service before terminating voluntarily. This parameter is ignored
by the Postfix queue manager and by other long-lived Postfix daemon
processes.

maximal_backoff_time (default: 4000s)
The maximal time between attempts to deliver a deferred message.

This parameter should be set to a value greater than or equal to $mini-
mal_backoff_time. See also $queue_run_delay.

maximal_queue_lifetime (default: 5d)
Consider a message as undeliverable, when delivery fails with a tempo-
rary error, and the time in the queue has reached the maxi-
mal_queue_lifetime limit.

Specify 0 when mail delivery should be tried only once.

message_drop_headers (default: bcc, content-length, resent-bcc, return-path)
Names of message headers that the cleanup(8) daemon will remove after
applying header_checks(5) and before invoking Milter applications. The
default setting is compatible with Postfix < 3.0.

Specify a list of header names, separated by comma or space. Names are
matched in a case-insensitive manner. The list of supported header
names is limited only by available memory.

This feature is available in Postfix 3.0 and later.

message_reject_characters (default: empty)
The set of characters that Postfix will reject in message content. The
usual C-like escape sequences are recognized: \a \b \f \n \r \t \v \ddd
(up to three octal digits) and \\.

Note 1: this feature does not recognize text that requires MIME decod-
ing. It inspects raw message content, just like header_checks and
body_checks.

Note 2: this feature is disabled with "receive_override_options =
no_header_body_checks".

Example:

message_reject_characters = \0

This feature is available in Postfix 2.3 and later.

message_size_limit (default: 10240000)
The maximal size in bytes of a message, including envelope information.
The value cannot exceed LONG_MAX (typically, a 32-bit or 64-bit signed
integer).

Note: be careful when making changes. Excessively small values will re-
sult in the loss of non-delivery notifications, when a bounce message
size exceeds the local or remote MTA's message size limit.

message_strip_characters (default: empty)
The set of characters that Postfix will remove from message content.
The usual C-like escape sequences are recognized: \a \b \f \n \r \t \v
\ddd (up to three octal digits) and \\.

Note 1: this feature does not recognize text that requires MIME decod-
ing. It inspects raw message content, just like header_checks and
body_checks.

Note 2: this feature is disabled with "receive_override_options =
no_header_body_checks".

Example:

message_strip_characters = \0

This feature is available in Postfix 2.3 and later.

meta_directory (default: see 'postconf -d' output)
The location of non-executable files that are shared among multiple
Postfix instances, such as postfix-files, dynamicmaps.cf, and the
multi-instance template files main.cf.proto and master.cf.proto. This
directory should contain only Postfix-related files. Typically, the
meta_directory parameter has the same default as the config_directory
parameter (/etc/postfix or /usr/local/etc/postfix).

For backwards compatibility with Postfix versions 2.6..2.11, specify
"meta_directory = $daemon_directory" in main.cf before installing or up-
grading Postfix, or specify "meta_directory = /path/name" on the "make
makefiles", "make install" or "make upgrade" command line.

This feature is available in Postfix 3.0 and later.

milter_command_timeout (default: 30s)
The time limit for sending an SMTP command to a Milter (mail filter) ap-
plication, and for receiving the response.

This feature is available in Postfix 2.3 and later.

milter_connect_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after com-
pletion of an SMTP connection. See MILTER_README for a list of available
macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_connect_timeout (default: 30s)
The time limit for connecting to a Milter (mail filter) application, and
for negotiating protocol options.

This feature is available in Postfix 2.3 and later.

milter_content_timeout (default: 300s)
The time limit for sending message content to a Milter (mail filter) ap-
plication, and for receiving the response.

This feature is available in Postfix 2.3 and later.

milter_data_macros (default: see postconf -d output)
The macros that are sent to version 4 or higher Milter (mail filter) ap-
plications after the SMTP DATA command. See MILTER_README for a list of
available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_default_action (default: tempfail)
The default action when a Milter (mail filter) response is unavailable
(for example, bad Postfix configuration or Milter failure). Specify one
of the following:

accept Proceed as if the mail filter was not present.

reject Reject all further commands in this session with a permanent sta-
tus code.

tempfail
Reject all further commands in this session with a temporary sta-
tus code.

quarantine
Like "accept", but freeze the message in the "hold" queue. Avail-
able with Postfix 2.6 and later.

This feature is available in Postfix 2.3 and later.

milter_end_of_data_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the
message end-of-data. See MILTER_README for a list of available macro
names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_end_of_header_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the
end of the message header. See MILTER_README for a list of available
macro names and their meanings.

This feature is available in Postfix 2.5 and later.

milter_header_checks (default: empty)
Optional lookup tables for content inspection of message headers that
are produced by Milter applications. See the header_checks(5) manual
page available actions. Currently, PREPEND is not implemented.

The following example sends all mail that is marked as SPAM to a spam
handling machine. Note that matches are case-insensitive by default.

/etc/postfix/main.cf:
milter_header_checks = pcre:/etc/postfix/milter_header_checks

/etc/postfix/milter_header_checks:
/^X-SPAM-FLAG:\s+YES/ FILTER mysmtp:sanitizer.example.com:25

The milter_header_checks mechanism could also be used for allowlisting.
For example it could be used to skip heavy content inspection for
DKIM-signed mail from known friendly domains.

This feature is available in Postfix 2.7, and as an optional patch for
Postfix 2.6.

milter_helo_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the
SMTP HELO or EHLO command. See MILTER_README for a list of available
macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_macro_daemon_name (default: $myhostname)
The {daemon_name} macro value for Milter (mail filter) applications.
See MILTER_README for a list of available macro names and their mean-
ings.

This feature is available in Postfix 2.3 and later.

milter_macro_defaults (default: empty)
Optional list of name=value pairs that specify default values for arbi-
trary macros that Postfix may send to Milter applications. These de-
faults are used when there is no corresponding information from the mes-
sage delivery context.

Specify name=value or {name=value} pairs separated by comma or white-
space. Enclose a pair in "{}" when a value contains comma or whitespace
(this form ignores whitespace after the enclosing "{", around the "=",
and before the enclosing "}").

This feature is available in Postfix 3.1 and later.

milter_macro_v (default: $mail_name $mail_version)
The {v} macro value for Milter (mail filter) applications. See MIL-
TER_README for a list of available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_mail_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the
SMTP MAIL FROM command. See MILTER_README for a list of available macro
names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_protocol (default: 6)
The mail filter protocol version and optional protocol extensions for
communication with a Milter application; prior to Postfix 2.6 the de-
fault protocol is 2. Postfix sends this version number during the ini-
tial protocol handshake. It should match the version number that is ex-
pected by the mail filter application (or by its Milter library).

Protocol versions:

2 Use Sendmail 8 mail filter protocol version 2 (default with Send-
mail version 8.11 .. 8.13 and Postfix version 2.3 .. 2.5).

3 Use Sendmail 8 mail filter protocol version 3.

4 Use Sendmail 8 mail filter protocol version 4.

6 Use Sendmail 8 mail filter protocol version 6 (default with Send-
mail version 8.14 and Postfix version 2.6).

Protocol extensions:

no_header_reply
Specify this when the Milter application will not reply for each
individual message header.

This feature is available in Postfix 2.3 and later.

milter_rcpt_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the
SMTP RCPT TO command. See MILTER_README for a list of available macro
names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_unknown_command_macros (default: see postconf -d output)
The macros that are sent to version 3 or higher Milter (mail filter) ap-
plications after an unknown SMTP command. See MILTER_README for a list
of available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

mime_boundary_length_limit (default: 2048)
The maximal length of MIME multipart boundary strings. The MIME proces-
sor is unable to distinguish between boundary strings that do not differ
in the first $mime_boundary_length_limit characters.

This feature is available in Postfix 2.0 and later.

mime_header_checks (default: $header_checks)
Optional lookup tables for content inspection of MIME related message
headers, as described in the header_checks(5) manual page.

This feature is available in Postfix 2.0 and later.

mime_nesting_limit (default: 100)
The maximal recursion level that the MIME processor will handle. Post-
fix refuses mail that is nested deeper than the specified limit.

This feature is available in Postfix 2.0 and later.

minimal_backoff_time (default: 300s)
The minimal time between attempts to deliver a deferred message; prior
to Postfix 2.4 the default value was 1000s.

This parameter also limits the time an unreachable destination is kept
in the short-term, in-memory, destination status cache.

This parameter should be set greater than or equal to $queue_run_delay.
See also $maximal_backoff_time.

multi_instance_directories (default: empty)
An optional list of non-default Postfix configuration directories; these
directories belong to additional Postfix instances that share the Post-
fix executable files and documentation with the default Postfix in-
stance, and that are started, stopped, etc., together with the default
Postfix instance. Specify a list of pathnames separated by comma or
whitespace.

When $multi_instance_directories is empty, the postfix(1) command runs
in single-instance mode and operates on a single Postfix instance only.
Otherwise, the postfix(1) command runs in multi-instance mode and in-
vokes the multi-instance manager specified with the multi_instance_wrap-
per parameter. The multi-instance manager in turn executes postfix(1)
commands for the default instance and for all Postfix instances in
$multi_instance_directories.

Currently, this parameter setting is ignored except for the default
main.cf file.

This feature is available in Postfix 2.6 and later.

multi_instance_enable (default: no)
Allow this Postfix instance to be started, stopped, etc., by a multi-in-
stance manager. By default, new instances are created in a safe state
that prevents them from being started inadvertently. This parameter is
reserved for the multi-instance manager.

This feature is available in Postfix 2.6 and later.

multi_instance_group (default: empty)
The optional instance group name of this Postfix instance. A group iden-
tifies closely-related Postfix instances that the multi-instance manager
can start, stop, etc., as a unit. This parameter is reserved for the
multi-instance manager.

This feature is available in Postfix 2.6 and later.

multi_instance_name (default: empty)
The optional instance name of this Postfix instance. This name becomes
also the default value for the syslog_name parameter.

This feature is available in Postfix 2.6 and later.

multi_instance_wrapper (default: empty)
The pathname of a multi-instance manager command that the postfix(1)
command invokes when the multi_instance_directories parameter value is
non-empty. The pathname may be followed by initial command arguments
separated by whitespace; shell metacharacters such as quotes are not
supported in this context.

The postfix(1) command invokes the manager command with the postfix(1)
non-option command arguments on the manager command line, and with all
installation configuration parameters exported into the manager command
process environment. The manager command in turn invokes the postfix(1)
command for individual Postfix instances as "postfix -c config_directory
command".

This feature is available in Postfix 2.6 and later.

multi_recipient_bounce_reject_code (default: 550)
The numerical Postfix SMTP server response code when a remote SMTP
client request is blocked by the reject_multi_recipient_bounce restric-
tion.

Do not change this unless you have a complete understanding of RFC 5321.

This feature is available in Postfix 2.1 and later.

mydestination (default: $myhostname, localhost.$mydomain, localhost)
The list of domains that are delivered via the $local_transport mail de-
livery transport. By default this is the Postfix local(8) delivery agent
which looks up all recipients in /etc/passwd and /etc/aliases. The SMTP
server validates recipient addresses with $local_recipient_maps and re-
jects non-existent recipients. See also the local domain class in the
ADDRESS_CLASS_README file.

The default mydestination value specifies names for the local machine
only. On a mail domain gateway, you should also include $mydomain.

The $local_transport delivery method is also selected for mail addressed
to user@[the.net.work.address] of the mail system (the IP addresses
specified with the inet_interfaces and proxy_interfaces parameters).

Warnings:

• Do not specify the names of virtual domains - those domains are
specified elsewhere. See VIRTUAL_README for more information.

• Do not specify the names of domains that this machine is backup
MX host for. See STANDARD_CONFIGURATION_README for how to set up
backup MX hosts.

• By default, the Postfix SMTP server rejects mail for recipients
not listed with the local_recipient_maps parameter. See the
postconf(5) manual for a description of the local_recipient_maps
and unknown_local_recipient_reject_code parameters.

Examples:

mydestination = $myhostname, localhost.$mydomain $mydomain
mydestination = $myhostname, localhost.$mydomain www.$mydomain, ftp.$mydomain

mydomain (default: see postconf -d output)
The internet domain name of this mail system. The default is to use
$myhostname minus the first component, or "localdomain" (Postfix 2.3 and
later). $mydomain is used as a default value for many other configura-
tion parameters.

Example:

mydomain = domain.tld

myhostname (default: see postconf -d output)
The internet hostname of this mail system. The default is to use the
fully-qualified domain name (FQDN) from gethostname(), or to use the
non-FQDN result from gethostname() and append ".$mydomain". $myhostname
is used as a default value for many other configuration parameters.

Example:

myhostname = host.example.com

mynetworks (default: see postconf -d output)
The list of "trusted" remote SMTP clients that have more privileges than
"strangers".

In particular, "trusted" SMTP clients are allowed to relay mail through
Postfix. See the smtpd_relay_restrictions parameter description in the
postconf(5) manual.

You can specify the list of "trusted" network addresses by hand or you
can let Postfix do it for you (which is the default). See the descrip-
tion of the mynetworks_style parameter for more information.

If you specify the mynetworks list by hand, Postfix ignores the mynet-
works_style setting.

Specify a list of network addresses or network/netmask patterns, sepa-
rated by commas and/or whitespace. Continue long lines by starting the
next line with whitespace.

The netmask specifies the number of bits in the network part of a host
address. You can also specify "/file/name" or "type:table" patterns. A
"/file/name" pattern is replaced by its contents; a "type:table" lookup
table is matched when a table entry matches a lookup string (the lookup
result is ignored).

The list is matched left to right, and the search stops on the first
match. Specify "!pattern" to exclude an address or network block from
the list. The form "!/file/name" is supported only in Postfix version
2.4 and later.

Note 1: Pattern matching of domain names is controlled by the presence
or absence of "mynetworks" in the parent_domain_matches_subdomains para-
meter value.

Note 2: IP version 6 address information must be specified inside [] in
the mynetworks value, and in files specified with "/file/name". IP ver-
sion 6 addresses contain the ":" character, and would otherwise be con-
fused with a "type:table" pattern.

Note 3: CIDR ranges cannot be specified in hash tables. Use cidr tables
if CIDR ranges are used.

Examples:

mynetworks = 127.0.0.0/8 168.100.189.0/28
mynetworks = !192.168.0.1, 192.168.0.0/28
mynetworks = 127.0.0.0/8 168.100.189.0/28 [::1]/128 [2001:240:587::]/64
mynetworks = $config_directory/mynetworks
mynetworks = hash:/etc/postfix/network_table
mynetworks = cidr:/etc/postfix/network_table.cidr

mynetworks_style (default: Postfix >= 3.0: host, Postfix < 3.0: subnet)
The method to generate the default value for the mynetworks parameter.
This is the list of trusted networks for relay access control etc.

• Specify "mynetworks_style = host" when Postfix should "trust"
only the local machine.

• Specify "mynetworks_style = subnet" when Postfix should "trust"
remote SMTP clients in the same IP subnetworks as the local ma-
chine. On Linux, this works correctly only with interfaces spec-
ified with the "ifconfig" or "ip" command.

• Specify "mynetworks_style = class" when Postfix should "trust"
remote SMTP clients in the same IP class A/B/C networks as the
local machine. Caution: this may cause Postfix to "trust" your
entire provider's network. Instead, specify an explicit mynet-
works list by hand, as described with the mynetworks configura-
tion parameter.

myorigin (default: $myhostname)
The domain name that locally-posted mail appears to come from, and that
locally posted mail is delivered to. The default, $myhostname, is ade-
quate for small sites. If you run a domain with multiple machines, you
should (1) change this to $mydomain and (2) set up a domain-wide alias
database that aliases each user to user@that.users.mailhost.

Example:

myorigin = $mydomain

nested_header_checks (default: $header_checks)
Optional lookup tables for content inspection of non-MIME message head-
ers in attached messages, as described in the header_checks(5) manual
page.

This feature is available in Postfix 2.0 and later.

newaliases_path (default: see postconf -d output)
Sendmail compatibility feature that specifies the location of the
newaliases(1) command. This command can be used to rebuild the local(8)
aliases(5) database.

non_fqdn_reject_code (default: 504)
The numerical Postfix SMTP server reply code when a client request is
rejected by the reject_non_fqdn_helo_hostname, reject_non_fqdn_sender or
reject_non_fqdn_recipient restriction.

non_smtpd_milters (default: empty)
A list of Milter (mail filter) applications for new mail that does not
arrive via the Postfix smtpd(8) server. This includes local submission
via the sendmail(1) command line, new mail that arrives via the Postfix
qmqpd(8) server, and old mail that is re-injected into the queue with
"postsuper -r". Specify space or comma as a separator. See the MIL-
TER_README document for details.

This feature is available in Postfix 2.3 and later.

notify_classes (default: resource, software)
The list of error classes that are reported to the postmaster. These
postmaster notifications do not replace user notifications. The default
is to report only the most serious problems. The paranoid may wish to
turn on the policy (UCE and mail relaying) and protocol error (broken
mail software) reports.

NOTE: postmaster notifications may contain confidential information such
as SASL passwords or message content. It is the system administrator's
responsibility to treat such information with care.

The error classes are:

bounce (also implies 2bounce)
Send the postmaster copies of the headers of bounced mail, and
send transcripts of SMTP sessions when Postfix rejects mail. The
notification is sent to the address specified with the bounce_no-
tice_recipient configuration parameter (default: postmaster).

2bounce
Send undeliverable bounced mail to the postmaster. The notifica-
tion is sent to the address specified with the 2bounce_notice_re-
cipient configuration parameter (default: postmaster).

data Send the postmaster a transcript of the SMTP session with an er-
ror because a critical data file was unavailable. The notifica-
tion is sent to the address specified with the error_notice_re-
cipient configuration parameter (default: postmaster).
This feature is available in Postfix 2.9 and later.

delay Send the postmaster copies of the headers of delayed mail (see
delay_warning_time). The notification is sent to the address
specified with the delay_notice_recipient configuration parameter
(default: postmaster).

policy Send the postmaster a transcript of the SMTP session when a
client request was rejected because of (UCE) policy. The notifi-
cation is sent to the address specified with the error_notice_re-
cipient configuration parameter (default: postmaster).

protocol
Send the postmaster a transcript of the SMTP session in case of
client or server protocol errors. The notification is sent to the
address specified with the error_notice_recipient configuration
parameter (default: postmaster).

resource
Inform the postmaster of mail not delivered due to resource prob-
lems. The notification is sent to the address specified with the
error_notice_recipient configuration parameter (default: postmas-
ter).

software
Inform the postmaster of mail not delivered due to software prob-
lems. The notification is sent to the address specified with the
error_notice_recipient configuration parameter (default: postmas-
ter).

Examples:

notify_classes = bounce, delay, policy, protocol, resource, software
notify_classes = 2bounce, resource, software

openssl_path (default: openssl)
The location of the OpenSSL command line program openssl(1). This is
used by the "postfix tls" command to create private keys, certificate
signing requests, self-signed certificates, and to compute public key
digests for DANE TLSA records. In multi-instance environments, this pa-
rameter is always determined from the configuration of the default Post-
fix instance.

Example:

/etc/postfix/main.cf:
# NetBSD pkgsrc:
openssl_path = /usr/pkg/bin/openssl
# Local build:
openssl_path = /usr/local/bin/openssl

This feature is available in Postfix 3.1 and later.

owner_request_special (default: yes)
Enable special treatment for owner-listname entries in the aliases(5)
file, and don't split owner-listname and listname-request address local-
parts when the recipient_delimiter is set to "-". This feature is use-
ful for mailing lists.

parent_domain_matches_subdomains (default: see postconf -d output)
A list of Postfix features where the pattern "example.com" also matches
subdomains of example.com, instead of requiring an explicit ".exam-
ple.com" pattern. This is planned backwards compatibility: eventually,
all Postfix features are expected to require explicit ".example.com"
style patterns when you really want to match subdomains.

The following Postfix feature names are supported.

Postfix version 1.0 and later
debug_peer_list, fast_flush_domains, mynetworks, per-
mit_mx_backup_networks, relay_domains, transport_maps

Postfix version 1.1 and later
qmqpd_authorized_clients, smtpd_access_maps,

Postfix version 2.8 and later
postscreen_access_list

Postfix version 3.0 and later
smtpd_client_event_limit_exceptions

permit_mx_backup_networks (default: empty)
Restrict the use of the permit_mx_backup SMTP access feature to only do-
mains whose primary MX hosts match the listed networks. The parameter
value syntax is the same as with the mynetworks parameter; note, how-
ever, that the default value is empty.

Pattern matching of domain names is controlled by the presence or ab-
sence of "permit_mx_backup_networks" in the parent_domain_matches_subdo-
mains parameter value.

pickup_service_name (default: pickup)
The name of the pickup(8) service. This service picks up local mail sub-
missions from the Postfix maildrop queue.

This feature is available in Postfix 2.0 and later.

pipe_delivery_status_filter (default: $default_delivery_status_filter)
Optional filter for the pipe(8) delivery agent to change the delivery
status code or explanatory text of successful or unsuccessful deliver-
ies. See default_delivery_status_filter for details.

This feature is available in Postfix 3.0 and later.

plaintext_reject_code (default: 450)
The numerical Postfix SMTP server response code when a request is re-
jected by the reject_plaintext_session restriction.

This feature is available in Postfix 2.3 and later.

postlog_service_name (default: postlog)
The name of the postlogd(8) service entry in master.cf. This service
appends logfile records to the file specified with the maillog_file pa-
rameter.

This feature is available in Postfix 3.4 and later.

postlogd_watchdog_timeout (default: 10s)
How much time a postlogd(8) process may take to process a request before
it is terminated by a built-in watchdog timer. This is a safety mecha-
nism that prevents postlogd(8) from becoming non-responsive due to a bug
in Postfix itself or in system software. This limit cannot be set under
10s.

This feature is available in Postfix 3.4 and later.

postmulti_control_commands (default: reload flush)
The postfix(1) commands that the postmulti(1) instance manager treats as
"control" commands, that operate on running instances. For these com-
mands, disabled instances are skipped.

This feature is available in Postfix 2.6 and later.

postmulti_start_commands (default: start)
The postfix(1) commands that the postmulti(1) instance manager treats as
"start" commands. For these commands, disabled instances are "checked"
rather than "started", and failure to "start" a member instance of an
instance group will abort the start-up of later instances.

This feature is available in Postfix 2.6 and later.

postmulti_stop_commands (default: see postconf -d output)
The postfix(1) commands that the postmulti(1) instance manager treats as
"stop" commands. For these commands, disabled instances are skipped, and
enabled instances are processed in reverse order.

This feature is available in Postfix 2.6 and later.

postscreen_access_list (default: permit_mynetworks)
Permanent allow/denylist for remote SMTP client IP addresses. post-
screen(8) searches this list immediately after a remote SMTP client con-
nects. Specify a comma- or whitespace-separated list of commands (in
upper or lower case) or lookup tables. The search stops upon the first
command that fires for the client IP address.

permit_mynetworks
Allowlist the client and terminate the search if the client IP
address matches $mynetworks. Do not subject the client to any
before/after 220 greeting tests. Pass the connection immediately
to a Postfix SMTP server process.
Pattern matching of domain names is controlled by the presence or
absence of "postscreen_access_list" in the parent_do-
main_matches_subdomains parameter value.

type:table
Query the specified lookup table. Each table lookup result is an
access list, except that access lists inside a table cannot spec-
ify type:table entries.
To discourage the use of hash, btree, etc. tables, there is no
support for substring matching like smtpd(8). Use CIDR tables in-
stead.

permit
Allowlist the client and terminate the search. Do not subject the
client to any before/after 220 greeting tests. Pass the connec-
tion immediately to a Postfix SMTP server process.

reject
Denylist the client and terminate the search. Subject the client
to the action configured with the postscreen_denylist_action con-
figuration parameter.

dunno All postscreen(8) access lists implicitly have this command at
the end.
When dunno is executed inside a lookup table, return from the
lookup table and evaluate the next command.
When dunno is executed outside a lookup table, terminate the
search, and subject the client to the configured before/after 220
greeting tests.

Example:

/etc/postfix/main.cf:
postscreen_access_list = permit_mynetworks,
cidr:/etc/postfix/postscreen_access.cidr
# Postfix < 3.6 use postscreen_blacklist_action.
postscreen_denylist_action = enforce

/etc/postfix/postscreen_access.cidr:
# Rules are evaluated in the order as specified.
# Denylist 192.168.* except 192.168.0.1.
192.168.0.1 dunno
192.168.0.0/16 reject

This feature is available in Postfix 2.8.

postscreen_allowlist_interfaces (default: static:all)
A list of local postscreen(8) server IP addresses where a non-al-
lowlisted remote SMTP client can obtain postscreen(8)'s temporary al-
lowlist status. This status is required before the client can talk to a
Postfix SMTP server process. By default, a client can obtain post-
screen(8)'s allowlist status on any local postscreen(8) server IP ad-
dress.

When postscreen(8) listens on both primary and backup MX addresses, the
postscreen_allowlist_interfaces parameter can be configured to give the
temporary allowlist status only when a client connects to a primary MX
address. Once a client is allowlisted it can talk to a Postfix SMTP
server on any address. Thus, clients that connect only to backup MX ad-
dresses will never become allowlisted, and will never be allowed to talk
to a Postfix SMTP server process.

Specify a list of network addresses or network/netmask patterns, sepa-
rated by commas and/or whitespace. The netmask specifies the number of
bits in the network part of a host address. Continue long lines by
starting the next line with whitespace.

You can also specify "/file/name" or "type:table" patterns. A
"/file/name" pattern is replaced by its contents; a "type:table" lookup
table is matched when a table entry matches a lookup string (the lookup
result is ignored).

The list is matched left to right, and the search stops on the first
match. Specify "!pattern" to exclude an address or network block from
the list.

Note: IP version 6 address information must be specified inside [] in
the postscreen_allowlist_interfaces value, and in files specified with
"/file/name". IP version 6 addresses contain the ":" character, and
would otherwise be confused with a "type:table" pattern.

Example:

/etc/postfix/main.cf:
# Don't allowlist connections to the backup IP address.
# Postfix < 3.6 use postscreen_whitelist_interfaces.
postscreen_allowlist_interfaces = !168.100.189.8, static:all

This feature is available in Postfix 3.6 and later.

Available as postscreen_whitelist_interfaces in Postfix 2.9 - 3.5.

postscreen_bare_newline_action (default: ignore)
The action that postscreen(8) takes when a remote SMTP client sends a
bare newline character, that is, a newline not preceded by carriage re-
turn. Specify one of the following:

ignore Ignore the failure of this test. Allow other tests to complete.
Do not repeat this test before the result from some other test
expires. This option is useful for testing and collecting sta-
tistics without blocking mail permanently.

enforce
Allow other tests to complete. Reject attempts to deliver mail
with a 550 SMTP reply, and log the helo/sender/recipient informa-
tion. Repeat this test the next time the client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat
this test the next time the client connects.

This feature is available in Postfix 2.8.

postscreen_bare_newline_enable (default: no)
Enable "bare newline" SMTP protocol tests in the postscreen(8) server.
These tests are expensive: a remote SMTP client must disconnect after it
passes the test, before it can talk to a real Postfix SMTP server.

This feature is available in Postfix 2.8.

postscreen_bare_newline_ttl (default: 30d)
The amount of time that postscreen(8) remembers that a client IP address
passed a "bare newline" SMTP protocol test, before it address is re-
quired to pass that test again. The default is long because a remote
SMTP client must disconnect after it passes the test, before it can talk
to a real Postfix SMTP server.

This feature is available in Postfix 2.8.

postscreen_blacklist_action (default: ignore)
Renamed to postscreen_denylist_action in Postfix 3.6.

This feature is available in Postfix 2.8 - 3.5.

postscreen_cache_cleanup_interval (default: 12h)
The amount of time between postscreen(8) cache cleanup runs. Cache
cleanup increases the load on the cache database and should therefore
not be run frequently. This feature requires that the cache database
supports the "delete" and "sequence" operators. Specify a zero interval
to disable cache cleanup.

After each cache cleanup run, the postscreen(8) daemon logs the number
of entries that were retained and dropped. A cleanup run is logged as
"partial" when the daemon terminates early after "postfix reload",
"postfix stop", or no requests for $max_idle seconds.

This feature is available in Postfix 2.8.

postscreen_cache_map (default: btree:$data_directory/postscreen_cache)
Persistent storage for the postscreen(8) server decisions.

To share a postscreen(8) cache between multiple postscreen(8) instances,
use "postscreen_cache_map = proxy:btree:/path/to/file". This requires
Postfix version 2.9 or later; earlier proxymap(8) implementations don't
support cache cleanup. For an alternative approach see the memcache_ta-
ble(5) manpage.

This feature is available in Postfix 2.8.

postscreen_cache_retention_time (default: 7d)
The amount of time that postscreen(8) will cache an expired temporary
allowlist entry before it is removed. This prevents clients from being
logged as "NEW" just because their cache entry expired an hour ago. It
also prevents the cache from filling up with clients that passed some
deep protocol test once and never came back.

This feature is available in Postfix 2.8.

postscreen_client_connection_count_limit (default: $smtpd_client_connec-
tion_count_limit)
How many simultaneous connections any remote SMTP client is allowed to
have with the postscreen(8) daemon. By default, this limit is the same
as with the Postfix SMTP server. Note that the triage process can take
several seconds, with the time spent in postscreen_greet_wait delay, and
with the time spent talking to the postscreen(8) built-in dummy SMTP
protocol engine.

This feature is available in Postfix 2.8.

postscreen_command_count_limit (default: 20)
The limit on the total number of commands per SMTP session for post-
screen(8)'s built-in SMTP protocol engine. This SMTP engine defers or
rejects all attempts to deliver mail, therefore there is no need to en-
force separate limits on the number of junk commands and error commands.

This feature is available in Postfix 2.8.

postscreen_command_filter (default: $smtpd_command_filter)
A mechanism to transform commands from remote SMTP clients. See
smtpd_command_filter for further details.

This feature is available in Postfix 2.8 and later.

postscreen_command_time_limit (default: normal: 300s, overload: 10s)
The time limit to read an entire command line with postscreen(8)'s
built-in SMTP protocol engine.

This feature is available in Postfix 2.8.

postscreen_denylist_action (default: ignore)
The action that postscreen(8) takes when a remote SMTP client is perma-
nently denylisted with the postscreen_access_list parameter. Specify
one of the following:

ignore (default)
Ignore this result. Allow other tests to complete. Repeat this
test the next time the client connects. This option is useful
for testing and collecting statistics without blocking mail.

enforce
Allow other tests to complete. Reject attempts to deliver mail
with a 550 SMTP reply, and log the helo/sender/recipient informa-
tion. Repeat this test the next time the client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat
this test the next time the client connects.

This feature is available in Postfix 3.6 and later.

Available as postscreen_blacklist_action in Postfix 2.8 - 3.5.

postscreen_disable_vrfy_command (default: $disable_vrfy_command)
Disable the SMTP VRFY command in the postscreen(8) daemon. See dis-
able_vrfy_command for details.

This feature is available in Postfix 2.8.

postscreen_discard_ehlo_keyword_address_maps (default: $smtpd_discard_ehlo_key-
word_address_maps)
Lookup tables, indexed by the remote SMTP client address, with case in-
sensitive lists of EHLO keywords (pipelining, starttls, auth, etc.) that
the postscreen(8) server will not send in the EHLO response to a remote
SMTP client. See smtpd_discard_ehlo_keywords for details. The table is
not searched by hostname for robustness reasons.

This feature is available in Postfix 2.8 and later.

postscreen_discard_ehlo_keywords (default: $smtpd_discard_ehlo_keywords)
A case insensitive list of EHLO keywords (pipelining, starttls, auth,
etc.) that the postscreen(8) server will not send in the EHLO response
to a remote SMTP client. See smtpd_discard_ehlo_keywords for details.

This feature is available in Postfix 2.8 and later.

postscreen_dnsbl_action (default: ignore)
The action that postscreen(8) takes when a remote SMTP client's combined
DNSBL score is equal to or greater than a threshold (as defined with the
postscreen_dnsbl_sites and postscreen_dnsbl_threshold parameters).
Specify one of the following:

ignore (default)
Ignore the failure of this test. Allow other tests to complete.
Repeat this test the next time the client connects. This option
is useful for testing and collecting statistics without blocking
mail.

enforce
Allow other tests to complete. Reject attempts to deliver mail
with a 550 SMTP reply, and log the helo/sender/recipient informa-
tion. Repeat this test the next time the client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat
this test the next time the client connects.

This feature is available in Postfix 2.8.

postscreen_dnsbl_allowlist_threshold (default: 0)
Allow a remote SMTP client to skip "before" and "after 220 greeting"
protocol tests, based on its combined DNSBL score as defined with the
postscreen_dnsbl_sites parameter.

Specify a negative value to enable this feature. When a client passes
the postscreen_dnsbl_allowlist_threshold without having failed other
tests, all pending or disabled tests are flagged as completed with an
expiration time based on the DNS reply TTL. When a test was already
completed, its expiration time is updated if it was less than the value
based on the DNS reply TTL. See also postscreen_dnsbl_max_ttl and post-
screen_dnsbl_min_ttl.

This feature is available in Postfix 3.6 and later.

Available as postscreen_dnsbl_whitelist_threshold in Postfix 2.11 - 3.5.

postscreen_dnsbl_max_ttl (default: ${postscreen_dnsbl_ttl?{$post-
screen_dnsbl_ttl}:{1}}h)
The maximum amount of time that postscreen(8) remembers that a client IP
address passed a DNS-based reputation test, before it is required to
pass that test again. If the DNS reply specifies a shorter TTL value,
that value will be used unless it would be smaller than post-
screen_dnsbl_min_ttl.

This feature is available in Postfix 3.1. The default setting is back-
wards-compatible with older Postfix versions.

postscreen_dnsbl_min_ttl (default: 60s)
The minimum amount of time that postscreen(8) remembers that a client IP
address passed a DNS-based reputation test, before it is required to
pass that test again. If the DNS reply specifies a larger TTL value,
that value will be used unless it would be larger than post-
screen_dnsbl_max_ttl.

This feature is available in Postfix 3.1.

postscreen_dnsbl_reply_map (default: empty)
A mapping from an actual DNSBL domain name which includes a secret pass-
word, to the DNSBL domain name that postscreen will reply with when it
rejects mail. When no mapping is found, the actual DNSBL domain will be
used.

For maximal stability it is best to use a file that is read into memory
such as pcre:, regexp: or texthash: (texthash: is similar to hash:, ex-
cept a) there is no need to run postmap(1) before the file can be used,
and b) texthash: does not detect changes after the file is read).

Example:

/etc/postfix/main.cf:
postscreen_dnsbl_reply_map = texthash:/etc/postfix/dnsbl_reply

/etc/postfix/dnsbl_reply:
secret.zen.spamhaus.org zen.spamhaus.org

NOTE: This feature differs from the Postfix SMTP server's rbl_reply_maps
feature, where 1) the table search key includes the optional "=ad-
dress-filter", and where 2) the lookup result contains free text with
$name variables.

This feature is available in Postfix 2.8.

postscreen_dnsbl_sites (default: empty)
Optional list of patterns with DNS allow/denylist domains, filters and
weight factors. When the list is non-empty, the dnsblog(8) daemon will
query these domains with the reversed IP addresses of remote SMTP
clients, and postscreen(8) will update an SMTP client's DNSBL score with
each non-error reply as described below.

Caution: when postscreen rejects mail, its SMTP response contains the
DNSBL domain name. Use the postscreen_dnsbl_reply_map feature to hide
"password" information in DNSBL domain names.

When a client's score is equal to or greater than the threshold speci-
fied with postscreen_dnsbl_threshold, postscreen(8) can drop the connec-
tion with the remote SMTP client.

Specify a list of domain=filter*weight patterns, separated by comma or
whitespace.

• When a pattern specifies no "=filter", postscreen(8) will use any
non-error DNSBL query result. Otherwise, postscreen(8) will use
only DNSBL query results that match the filter. The filter has
the form d.d.d.d, where each d is a number, or a pattern inside
[] that contains one or more ";"-separated numbers or num-
ber..number ranges.

• When a pattern specifies no "*weight", the weight of the pattern
is 1. Otherwise, the weight must be an integral number. Specify
a negative number for allowlisting.

• When a pattern matches one or more DNSBL query results, post-
screen(8) adds that pattern's weight once to the remote SMTP
client's DNSBL score.

Examples:

To use example.com as a high-confidence blocklist, and to block mail
with example.net and example.org only when both agree:

postscreen_dnsbl_threshold = 2
postscreen_dnsbl_sites = example.com*2, example.net, example.org

To filter only DNSBL replies containing 127.0.0.4:

postscreen_dnsbl_sites = example.com=127.0.0.4

This feature is available in Postfix 2.8.

postscreen_dnsbl_threshold (default: 1)
The inclusive lower bound for blocking a remote SMTP client, based on
its combined DNSBL score as defined with the postscreen_dnsbl_sites pa-
rameter.

This feature is available in Postfix 2.8.

postscreen_dnsbl_timeout (default: 10s)
The time limit for DNSBL or DNSWL lookups. This is separate from the
timeouts in the dnsblog(8) daemon which are defined by system re-
solver(3) routines.

This feature is available in Postfix 3.0.

postscreen_dnsbl_ttl (default: 1h)
The amount of time that postscreen(8) remembers that a client IP address
passed a DNS-based reputation test, before it is required to pass that
test again.

This feature is available in Postfix 2.8-3.0. It was replaced by post-
screen_dnsbl_max_ttl in Postfix 3.1.

postscreen_dnsbl_whitelist_threshold (default: 0)
Renamed to postscreen_dnsbl_allowlist_threshold in Postfix 3.6.

This feature is available in Postfix 2.11 - 3.5.

postscreen_enforce_tls (default: $smtpd_enforce_tls)
Mandatory TLS: announce STARTTLS support to remote SMTP clients, and re-
quire that clients use TLS encryption. See smtpd_postscreen_enforce_tls
for details.

This feature is available in Postfix 2.8 and later. Preferably, use
postscreen_tls_security_level instead.

postscreen_expansion_filter (default: see postconf -d output)
List of characters that are permitted in postscreen_reject_footer at-
tribute expansions. See smtpd_expansion_filter for further details.

This feature is available in Postfix 2.8 and later.

postscreen_forbidden_commands (default: $smtpd_forbidden_commands)
List of commands that the postscreen(8) server considers in violation of
the SMTP protocol. See smtpd_forbidden_commands for syntax, and post-
screen_non_smtp_command_action for possible actions.

This feature is available in Postfix 2.8.

postscreen_greet_action (default: ignore)
The action that postscreen(8) takes when a remote SMTP client speaks be-
fore its turn within the time specified with the postscreen_greet_wait
parameter. Specify one of the following:

enforce
Allow other tests to complete. Reject attempts to deliver mail
with a 550 SMTP reply, and log the helo/sender/recipient informa-
tion. Repeat this test the next time the client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat
this test the next time the client connects.

In either case, postscreen(8) will not allowlist the remote SMTP client
IP address.

This feature is available in Postfix 2.8.

postscreen_greet_banner (default: $smtpd_banner)
The text in the optional "220-text..." server response that post-
screen(8) sends ahead of the real Postfix SMTP server's "220 text..."
response, in an attempt to confuse bad SMTP clients so that they speak
before their turn (pre-greet). Specify an empty value to disable this
feature.

This feature is available in Postfix 2.8.

postscreen_greet_ttl (default: 1d)
The amount of time that postscreen(8) remembers that a client IP address
passed a PREGREET test, before it is required to pass that test again.
The default is relatively short, because a good client can immediately
talk to a real Postfix SMTP server.

This feature is available in Postfix 2.8.

postscreen_greet_wait (default: normal: 6s, overload: 2s)
The amount of time that postscreen(8) will wait for an SMTP client to
send a command before its turn, and for DNS blocklist lookup results to
arrive (default: up to 2 seconds under stress, up to 6 seconds other-
wise).

This feature is available in Postfix 2.8.

postscreen_helo_required (default: $smtpd_helo_required)
Require that a remote SMTP client sends HELO or EHLO before commencing a
MAIL transaction.

This feature is available in Postfix 2.8.

postscreen_non_smtp_command_action (default: drop)
The action that postscreen(8) takes when a remote SMTP client sends
non-SMTP commands as specified with the postscreen_forbidden_commands
parameter. Specify one of the following:

enforce
Allow other tests to complete. Reject attempts to deliver mail
with a 550 SMTP reply, and log the helo/sender/recipient informa-
tion. Repeat this test the next time the client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat
this test the next time the client connects. This action is the
same as with the Postfix SMTP server's smtpd_forbidden_commands
feature.

This feature is available in Postfix 2.8.

postscreen_non_smtp_command_enable (default: no)
Enable "non-SMTP command" tests in the postscreen(8) server. These tests
are expensive: a client must disconnect after it passes the test, before
it can talk to a real Postfix SMTP server.

This feature is available in Postfix 2.8.

postscreen_non_smtp_command_ttl (default: 30d)
The amount of time that postscreen(8) remembers that a client IP address
passed a "non_smtp_command" SMTP protocol test, before it is required to
pass that test again. The default is long because a client must discon-
nect after it passes the test, before it can talk to a real Postfix SMTP
server.

This feature is available in Postfix 2.8.

postscreen_pipelining_action (default: enforce)
The action that postscreen(8) takes when a remote SMTP client sends mul-
tiple commands instead of sending one command and waiting for the server
to respond. Specify one of the following:

enforce
Allow other tests to complete. Reject attempts to deliver mail
with a 550 SMTP reply, and log the helo/sender/recipient informa-
tion. Repeat this test the next time the client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat
this test the next time the client connects.

This feature is available in Postfix 2.8.

postscreen_pipelining_enable (default: no)
Enable "pipelining" SMTP protocol tests in the postscreen(8) server.
These tests are expensive: a good client must disconnect after it passes
the test, before it can talk to a real Postfix SMTP server.

This feature is available in Postfix 2.8.

postscreen_pipelining_ttl (default: 30d)
The amount of time that postscreen(8) remembers that a client IP address
passed a "pipelining" SMTP protocol test, before it is required to pass
that test again. The default is long because a good client must discon-
nect after it passes the test, before it can talk to a real Postfix SMTP
server.

This feature is available in Postfix 2.8.

postscreen_post_queue_limit (default: $default_process_limit)
The number of clients that can be waiting for service from a real Post-
fix SMTP server process. When this queue is full, all clients will re-
ceive a 421 response.

This feature is available in Postfix 2.8.

postscreen_pre_queue_limit (default: $default_process_limit)
The number of non-allowlisted clients that can be waiting for a decision
whether they will receive service from a real Postfix SMTP server
process. When this queue is full, all non-allowlisted clients will re-
ceive a 421 response.

This feature is available in Postfix 2.8.

postscreen_reject_footer (default: $smtpd_reject_footer)
Optional information that is appended after a 4XX or 5XX postscreen(8)
server response. See smtpd_reject_footer for further details.

This feature is available in Postfix 2.8 and later.

postscreen_reject_footer_maps (default: $smtpd_reject_footer_maps)
Optional lookup table for information that is appended after a 4XX or
5XX postscreen(8) server response. See smtpd_reject_footer_maps for fur-
ther details.

This feature is available in Postfix 3.4 and later.

postscreen_tls_security_level (default: $smtpd_tls_security_level)
The SMTP TLS security level for the postscreen(8) server; when a
non-empty value is specified, this overrides the obsolete parameters
postscreen_use_tls and postscreen_enforce_tls. See smtpd_tls_secu-
rity_level for details.

This feature is available in Postfix 2.8 and later.

postscreen_upstream_proxy_protocol (default: empty)
The name of the proxy protocol used by an optional before-postscreen
proxy agent. When a proxy agent is used, this protocol conveys local and
remote address and port information. Specify "postscreen_up-
stream_proxy_protocol = haproxy" to enable the haproxy protocol; version
2 is supported with Postfix 3.5 and later.

This feature is available in Postfix 2.10 and later.

postscreen_upstream_proxy_timeout (default: 5s)
The time limit for the proxy protocol specified with the postscreen_up-
stream_proxy_protocol parameter.

This feature is available in Postfix 2.10 and later.

postscreen_use_tls (default: $smtpd_use_tls)
Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but
do not require that clients use TLS encryption.

This feature is available in Postfix 2.8 and later. Preferably, use
postscreen_tls_security_level instead.

postscreen_watchdog_timeout (default: 10s)
How much time a postscreen(8) process may take to respond to a remote
SMTP client command or to perform a cache operation before it is termi-
nated by a built-in watchdog timer. This is a safety mechanism that
prevents postscreen(8) from becoming non-responsive due to a bug in
Postfix itself or in system software. To avoid false alarms and unnec-
essary cache corruption this limit cannot be set under 10s.

This feature is available in Postfix 2.8.

postscreen_whitelist_interfaces (default: static:all)
Renamed to postscreen_allowlist_interfaces in Postfix 3.6.

This feature is available in Postfix 2.9 - 3.5.

prepend_delivered_header (default: command, file, forward)
The message delivery contexts where the Postfix local(8) delivery agent
prepends a Delivered-To: message header with the address that the mail
was delivered to. This information is used for mail delivery loop detec-
tion.

By default, the Postfix local delivery agent prepends a Delivered-To:
header when forwarding mail and when delivering to file (mailbox) and
command. Turning off the Delivered-To: header when forwarding mail is
not recommended.

Specify zero or more of forward, file, or command.

Example:

prepend_delivered_header = forward

process_id (read-only)
The process ID of a Postfix command or daemon process.

process_id_directory (default: pid)
The location of Postfix PID files relative to $queue_directory. This is
a read-only parameter.

process_name (read-only)
The process name of a Postfix command or daemon process.

propagate_unmatched_extensions (default: canonical, virtual)
What address lookup tables copy an address extension from the lookup key
to the lookup result.

For example, with a virtual(5) mapping of "joe@example.com =>
joe.user@example.net", the address "joe+foo@example.com" would rewrite
to "joe.user+foo@example.net".

Specify zero or more of canonical, virtual, alias, forward, include or
generic. These cause address extension propagation with canonical(5),
virtual(5), and aliases(5) maps, with local(8) .forward and :include:
file lookups, and with smtp(8) generic maps, respectively.

Note: enabling this feature for types other than canonical and virtual
is likely to cause problems when mail is forwarded to other sites, espe-
cially with mail that is sent to a mailing list exploder address.

Examples:

propagate_unmatched_extensions = canonical, virtual, alias,
forward, include
propagate_unmatched_extensions = canonical, virtual

proxy_interfaces (default: empty)
The remote network interface addresses that this mail system receives
mail on by way of a proxy or network address translation unit.

This feature is available in Postfix 2.0 and later.

You must specify your "outside" proxy/NAT addresses when your system is
a backup MX host for other domains, otherwise mail delivery loops will
happen when the primary MX host is down.

Example:

proxy_interfaces = 1.2.3.4

proxy_read_maps (default: see postconf -d output)
The lookup tables that the proxymap(8) server is allowed to access for
the read-only service.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Table references that don't begin with proxy: are ignored.

This feature is available in Postfix 2.0 and later.

proxy_write_maps (default: see postconf -d output)
The lookup tables that the proxymap(8) server is allowed to access for
the read-write service. Postfix-owned local database files should be
stored under the Postfix-owned data_directory. Table references that
don't begin with proxy: are ignored.

This feature is available in Postfix 2.5 and later.

proxymap_service_name (default: proxymap)
The name of the proxymap read-only table lookup service. This service
is normally implemented by the proxymap(8) daemon.

This feature is available in Postfix 2.6 and later.

proxywrite_service_name (default: proxywrite)
The name of the proxywrite read-write table lookup service. This ser-
vice is normally implemented by the proxymap(8) daemon.

This feature is available in Postfix 2.6 and later.

qmgr_clog_warn_time (default: 300s)
The minimal delay between warnings that a specific destination is clog-
ging up the Postfix active queue. Specify 0 to disable.

This feature is enabled with the helpful_warnings parameter.

This feature is available in Postfix 2.0 and later.

qmgr_daemon_timeout (default: 1000s)
How much time a Postfix queue manager process may take to handle a re-
quest before it is terminated by a built-in watchdog timer.

This feature is available in Postfix 2.8 and later.

qmgr_fudge_factor (default: 100)
Obsolete feature: the percentage of delivery resources that a busy mail
system will use up for delivery of a large mailing list message.

This feature exists only in the oqmgr(8) old queue manager. The current
queue manager solves the problem in a better way.

qmgr_ipc_timeout (default: 60s)
The time limit for the queue manager to send or receive information over
an internal communication channel. The purpose is to break out of dead-
lock situations. If the time limit is exceeded the software either re-
tries or aborts the operation.

This feature is available in Postfix 2.8 and later.

qmgr_message_active_limit (default: 20000)
The maximal number of messages in the active queue.

qmgr_message_recipient_limit (default: 20000)
The maximal number of recipients held in memory by the Postfix queue
manager, and the maximal size of the short-term, in-memory "dead" desti-
nation status cache.

qmgr_message_recipient_minimum (default: 10)
The minimal number of in-memory recipients for any message. This takes
priority over any other in-memory recipient limits (i.e., the global
qmgr_message_recipient_limit and the per transport _recipient_limit) if
necessary. The minimum value allowed for this parameter is 1.

qmqpd_authorized_clients (default: empty)
What remote QMQP clients are allowed to connect to the Postfix QMQP
server port.

By default, no client is allowed to use the service. This is because the
QMQP server will relay mail to any destination.

Specify a list of client patterns. A list pattern specifies a host name,
a domain name, an internet address, or a network/mask pattern, where the
mask specifies the number of bits in the network part. When a pattern
specifies a file name, its contents are substituted for the file name;
when a pattern is a "type:table" table specification, table lookup is
used instead.

Patterns are separated by whitespace and/or commas. In order to reverse
the result, precede a pattern with an exclamation point (!). The form
"!/file/name" is supported only in Postfix version 2.4 and later.

Pattern matching of domain names is controlled by the presence or ab-
sence of "qmqpd_authorized_clients" in the parent_domain_matches_subdo-
mains parameter value.

Example:

qmqpd_authorized_clients = !192.168.0.1, 192.168.0.0/24

qmqpd_client_port_logging (default: no)
Enable logging of the remote QMQP client port in addition to the host-
name and IP address. The logging format is "host[address]:port".

This feature is available in Postfix 2.5 and later.

qmqpd_error_delay (default: 1s)
How long the Postfix QMQP server will pause before sending a negative
reply to the remote QMQP client. The purpose is to slow down confused or
malicious clients.

qmqpd_timeout (default: 300s)
The time limit for sending or receiving information over the network.
If a read or write operation blocks for more than $qmqpd_timeout seconds
the Postfix QMQP server gives up and disconnects.

queue_directory (default: see postconf -d output)
The location of the Postfix top-level queue directory. This is the root
directory of Postfix daemon processes that run chrooted.

queue_file_attribute_count_limit (default: 100)
The maximal number of (name=value) attributes that may be stored in a
Postfix queue file. The limit is enforced by the cleanup(8) server.

This feature is available in Postfix 2.0 and later.

queue_minfree (default: 0)
The minimal amount of free space in bytes in the queue file system that
is needed to receive mail. This is currently used by the Postfix SMTP
server to decide if it will accept any mail at all.

By default, the Postfix SMTP server rejects MAIL FROM commands when the
amount of free space is less than 1.5*$message_size_limit (Postfix ver-
sion 2.1 and later). To specify a higher minimum free space limit,
specify a queue_minfree value that is at least 1.5*$message_size_limit.

With Postfix versions 2.0 and earlier, a queue_minfree value of zero
means there is no minimum required amount of free space.

queue_run_delay (default: 300s)
The time between deferred queue scans by the queue manager; prior to
Postfix 2.4 the default value was 1000s.

This parameter should be set less than or equal to $minimal_back-
off_time. See also $maximal_backoff_time.

queue_service_name (default: qmgr)
The name of the qmgr(8) service. This service manages the Postfix queue
and schedules delivery requests.

This feature is available in Postfix 2.0 and later.

rbl_reply_maps (default: empty)
Optional lookup tables with RBL or RHSBL response templates. The table
search key is the reject_rbl_* or reject_rhsbl_* argument, including any
optional "=address-filter". With Postfix 3.10 and later, if the result
is "not found" and the search key has the form domain=address-filter,
then rbl_reply_maps will also search with the domain.

By default, Postfix uses the template as specified with the de-
fault_rbl_reply configuration parameter. See there for a discussion of
the response template syntax.

Example:

/etc/postfix/main.cf:
rbl_reply_maps = hash:/etc/postfix/rbl_reply
smtpd_recipient_restrictions =
permit_mynetworks,
reject_rbl_client secret.zen.dq.spamhaus.net=127.0.0.[2..11],
...
/etc/postfix/rbl_reply:
# See https://www.postfix.org/postconf.5.html#default_rbl_reply
# for the definition of the $client_address and $rbl_class etc.
# variables.
secret.zen.dq.spamhaus.net=127.0.0.[2..11]
554 $rbl_class $rbl_what blocked using ZEN - see https://www.spamhaus.org/query/ip/$client_address for details

# Postfix >= 3.10: if a search key domain=address-filter
# is not found, then rbl_reply_maps will also search with the
# domain.
secret.zen.dq.spamhaus.net
554 $rbl_class $rbl_what blocked using ZEN - see https://www.spamhaus.org/query/ip/$client_address for details

NOTE: This feature differs from postscreen_dnsbl_reply_map where the ta-
ble search key is only a domain name (no "=address-filter", no "*weight"
factor) and where the lookup result should be only a domain name (no
free text, no $name variables).

This feature is available in Postfix 2.0 and later. The "=address-fil-
ter" feature is available in Postfix 2.8 and later.

readme_directory (default: see postconf -d output)
The location of Postfix README files that describe how to build, config-
ure or operate a specific Postfix subsystem or feature.

receive_override_options (default: empty)
Enable or disable recipient validation, built-in content filtering, or
address mapping. Typically, these are specified in master.cf as com-
mand-line arguments for the smtpd(8), qmqpd(8) or pickup(8) daemons.

Specify zero or more of the following options. The options override
main.cf settings and are either implemented by smtpd(8), qmqpd(8), or
pickup(8) themselves, or they are forwarded to the cleanup server.

no_unknown_recipient_checks
Do not try to reject unknown recipients (SMTP server only). This
is typically specified AFTER an external content filter.

no_address_mappings
Disable canonical address mapping, virtual alias map expansion,
address masquerading, and automatic BCC (blind carbon-copy) re-
cipients. This is typically specified BEFORE an external content
filter.

no_header_body_checks
Disable header/body_checks. This is typically specified AFTER an
external content filter.

no_milters
Disable Milter (mail filter) applications. This is typically
specified AFTER an external content filter.

Note: when the "BEFORE content filter" receive_override_options setting
is specified in the main.cf file, specify the "AFTER content filter" re-
ceive_override_options setting in master.cf (and vice versa).

Examples:

receive_override_options =
no_unknown_recipient_checks, no_header_body_checks
receive_override_options = no_address_mappings

This feature is available in Postfix 2.1 and later.

recipient_bcc_maps (default: empty)
Optional BCC (blind carbon-copy) address lookup tables, indexed by enve-
lope recipient address. The BCC address (multiple results are not sup-
ported) is added when mail enters from outside of Postfix.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

The table search order is as follows:

• Look up the "user+extension@domain.tld" address including the op-
tional address extension.

• Look up the "user@domain.tld" address without the optional ad-
dress extension.

• Look up the "user+extension" address local part when the recipi-
ent domain equals $myorigin, $mydestination, $inet_interfaces or
$proxy_interfaces.

• Look up the "user" address local part when the recipient domain
equals $myorigin, $mydestination, $inet_interfaces or $proxy_in-
terfaces.

• Look up the "@domain.tld" part.

Note: with Postfix 2.2 and earlier the sender will unconditionally be
notified when the BCC address is undeliverable.

Note: automatic BCC recipients are subject to address canonicalization
(add missing domain), canonical_maps, masquerade_domains, and vir-
tual_alias_maps.

Example:

recipient_bcc_maps = hash:/etc/postfix/recipient_bcc

After a change, run "postmap /etc/postfix/recipient_bcc".

This feature is available in Postfix 2.1 and later.

recipient_canonical_classes (default: envelope_recipient, header_recipient)
What addresses are subject to recipient_canonical_maps address mapping.
By default, recipient_canonical_maps address mapping is applied to enve-
lope recipient addresses, and to header recipient addresses.

Specify one or more of: envelope_recipient, header_recipient

This feature is available in Postfix 2.2 and later.

recipient_canonical_maps (default: empty)
Optional address mapping lookup tables for envelope and header recipient
addresses. The table format and lookups are documented in canonical(5).

Note: $recipient_canonical_maps is processed before $canonical_maps.

Example:

recipient_canonical_maps = hash:/etc/postfix/recipient_canonical

recipient_delimiter (default: empty)
The set of characters that can separate an email address localpart, user
name, or a .forward file name from its extension. For example, with
"recipient_delimiter = +", the software tries user+foo@example.com be-
fore trying user@example.com, user+foo before trying user, and .for-
ward+foo before trying .forward.

More formally, an email address localpart or user name is separated from
its extension by the first character that matches the recipient_delim-
iter set. The delimiter character and extension may then be used to gen-
erate an extended .forward file name. This implementation recognizes one
delimiter character and one extension per email address localpart or
email address. With Postfix 2.10 and earlier, the recipient_delimiter
specifies a single character.

See canonical(5), local(8), relocated(5) and virtual(5) for the effects
of recipient_delimiter on lookups in aliases, canonical, virtual, and
relocated maps, and see the propagate_unmatched_extensions parameter for
propagating an extension from one email address to another.

When used in command_execution_directory, forward_path, or luser_relay,
${recipient_delimiter} is replaced with the actual recipient delimiter
that was found in the recipient email address (Postfix 2.11 and later),
or it is replaced with the main.cf recipient_delimiter parameter value
(Postfix 2.10 and earlier).

The recipient_delimiter is not applied to the mailer-daemon address, the
postmaster address, or the double-bounce address. With the default
"owner_request_special = yes" setting, the recipient_delimiter is also
not applied to addresses with the special "owner-" prefix or the special
"-request" suffix.

Examples:

# Handle Postfix-style extensions.
recipient_delimiter = +

# Handle both Postfix and qmail extensions (Postfix 2.11 and later).
recipient_delimiter = +-

# Use .forward for mail without address extension, and for mail with
# an unrecognized address extension.
forward_path = $home/.forward${recipient_delimiter}${extension},
$home/.forward

reject_code (default: 554)
The numerical Postfix SMTP server response code when a remote SMTP
client request is rejected by the "reject" restriction.

Do not change this unless you have a complete understanding of RFC 5321.

reject_tempfail_action (default: defer_if_permit)
The Postfix SMTP server's action when a reject-type restriction fails
due to a temporary error condition. Specify "defer" to defer the remote
SMTP client request immediately. With the default "defer_if_permit" ac-
tion, the Postfix SMTP server continues to look for opportunities to re-
ject mail, and defers the client request only if it would otherwise be
accepted.

For finer control, see: unverified_recipient_tempfail_action, unveri-
fied_sender_tempfail_action, unknown_address_tempfail_action, and un-
known_helo_hostname_tempfail_action.

This feature is available in Postfix 2.6 and later.

relay_clientcerts (default: empty)
List of tables with remote SMTP client-certificate fingerprints or pub-
lic key fingerprints (Postfix 2.9 and later) for which the Postfix SMTP
server will allow access with the permit_tls_clientcerts feature. The
fingerprint digest algorithm is configurable via the smtpd_tls_finger-
print_digest parameter (hard-coded as md5 prior to Postfix version 2.5).

The default algorithm is sha256 with Postfix >= 3.6 and the compatibil-
ity_level set to 3.6 or higher. With Postfix <= 3.5, the default algo-
rithm is md5. The best-practice algorithm is now sha256. Recent ad-
vances in hash function cryptanalysis have led to md5 and sha1 being
deprecated in favor of sha256. However, as long as there are no known
"second pre-image" attacks against the older algorithms, their use in
this context, though not recommended, is still likely safe.

Postfix lookup tables are in the form of (key, value) pairs. Since we
only need the key, the value can be chosen freely, e.g. the name of the
user or host: D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80
lutzpc.at.home

Example:

relay_clientcerts = hash:/etc/postfix/relay_clientcerts

For more fine-grained control, use check_ccert_access to select an ap-
propriate access(5) policy for each client. See RESTRIC-
TION_CLASS_README.

This feature is available with Postfix version 2.2.

relay_destination_concurrency_limit (default: $default_destination_concur-
rency_limit)
The maximal number of parallel deliveries to the same destination via
the relay message delivery transport. This limit is enforced by the
queue manager. The message delivery transport name is the first field in
the entry in the master.cf file.

This feature is available in Postfix 2.0 and later.

relay_destination_recipient_limit (default: $default_destination_recipi-
ent_limit)
The maximal number of recipients per message for the relay message de-
livery transport. This limit is enforced by the queue manager. The mes-
sage delivery transport name is the first field in the entry in the mas-
ter.cf file.

Setting this parameter to a value of 1 changes the meaning of relay_des-
tination_concurrency_limit from concurrency per domain into concurrency
per recipient.

This feature is available in Postfix 2.0 and later.

relay_domains (default: Postfix >= 3.0: empty, Postfix < 3.0: $mydestination)
What destination domains (and subdomains thereof) this system will relay
mail to. For details about how the relay_domains value is used, see the
description of the permit_auth_destination and reject_unauth_destination
SMTP recipient restrictions.

Domains that match $relay_domains are delivered with the $relay_trans-
port mail delivery transport. The SMTP server validates recipient ad-
dresses with $relay_recipient_maps and rejects non-existent recipients.
See also the relay domains address class in the ADDRESS_CLASS_README
file.

Note: Postfix will not automatically forward mail for domains that list
this system as their primary or backup MX host. See the permit_mx_backup
restriction in the postconf(5) manual page.

Specify a list of host or domain names, "/file/name" patterns or
"type:table" lookup tables, separated by commas and/or whitespace. Con-
tinue long lines by starting the next line with whitespace. A
"/file/name" pattern is replaced by its contents; a "type:table" lookup
table is matched when a (parent) domain appears as lookup key. Specify
"!pattern" to exclude a domain from the list. The form "!/file/name" is
supported only in Postfix version 2.4 and later.

Pattern matching of domain names is controlled by the presence or ab-
sence of "relay_domains" in the parent_domain_matches_subdomains parame-
ter value.

relay_domains_reject_code (default: 554)
The numerical Postfix SMTP server response code when a client request is
rejected by the reject_unauth_destination recipient restriction.

Do not change this unless you have a complete understanding of RFC 5321.

relay_recipient_maps (default: empty)
Optional lookup tables with all valid addresses in the domains that
match $relay_domains. Specify @domain as a wild-card for domains that
have no valid recipient list, and become a source of backscatter mail:
Postfix accepts spam for non-existent recipients and then floods inno-
cent people with undeliverable mail. Technically, tables listed with
$relay_recipient_maps are used as lists: Postfix needs to know only if a
lookup string is found or not, but it does not use the result from the
table lookup.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

If this parameter is non-empty, then the Postfix SMTP server will reject
mail to unknown relay users. This feature is off by default.

See also the relay domains address class in the ADDRESS_CLASS_README
file.

Example:

relay_recipient_maps = hash:/etc/postfix/relay_recipients

This feature is available in Postfix 2.0 and later.

relay_transport (default: relay)
The default mail delivery transport and next-hop destination for the re-
lay domain address class: recipient domains that match $relay_domains.

For recipient domains in the relay domain address class:

• In order of decreasing precedence, the message delivery transport
is taken from 1) $transport_maps, 2) $relay_transport.

• In order of decreasing precedence, the nexthop destination is
taken from 1) $transport_maps, 2) $relay_transport, 3)
$sender_dependent_relayhost_maps or $relayhost or the recipient
domain.

See also the relay domains address class in the ADDRESS_CLASS_README
file.

This feature is available in Postfix 2.0 and later.

relayhost (default: empty)
The next-hop destination(s) for non-local mail; takes precedence over
non-local domains in recipient addresses. This information will not be
used when the sender matches $sender_dependent_relayhost_maps.

In order of decreasing precedence:

• For recipient domains in the relay domain address class (domains
matching $relay_domains), the nexthop destination is taken from
1) $transport_maps, 2) $relay_transport, 3) $sender_dependent_re-
layhost_maps or $relayhost or the recipient domain.

• For recipient domains in the default domain address class (do-
mains that do not match $mydestination, $inet_interfaces,
$proxy_interfaces, $virtual_alias_domains, $virtual_mailbox_do-
mains, or $relay_domains), the nexthop destination is taken from
1) $transport_maps, 2) $sender_dependent_default_transport_maps
or $default_transport, 3) $sender_dependent_relayhost_maps or
$relayhost or the recipient domain.

On an intranet, specify the organizational domain name. If your internal
DNS uses no MX records, specify the name of the intranet gateway host
instead.

In the case of SMTP delivery, specify one or more destinations in the
form of a domain name, hostname, hostname:service, [hostname]:service,
[hostaddress] or [hostaddress]:service, separated by comma or white-
space. The form [hostname] turns off MX or SRV lookups. Multiple desti-
nations are supported in Postfix 3.5 and later. Each destination is
tried in the specified order.

If an SMTP destination is a load balancer, and there are no alternative
destinations, specify the load balancer multiple times. Without the du-
plicate info, the Postfix SMTP client would not reconnect immediately to
the same load balancer after a remote SMTP server failure.

If you're connected via UUCP, see the UUCP_README file for useful infor-
mation.

Examples:

relayhost = $mydomain
relayhost = [gateway.example.com]
relayhost = mail1.example:587, mail2.example:587
relayhost = [an.ip.add.ress]

relocated_maps (default: empty)
Optional lookup tables with new contact information for users or domains
that no longer exist. The table format and lookups are documented in
relocated(5).

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

If you use this feature, run "postmap /etc/postfix/relocated" to build
the necessary DBM or DB file after change, then "postfix reload" to make
the changes visible.

Examples:

relocated_maps = dbm:/etc/postfix/relocated
relocated_maps = hash:/etc/postfix/relocated

remote_header_rewrite_domain (default: empty)
Rewrite or add message headers in mail from remote clients if the re-
mote_header_rewrite_domain parameter value is non-empty, updating incom-
plete addresses with the domain specified in the re-
mote_header_rewrite_domain parameter, and adding missing headers.

The local_header_rewrite_clients parameter controls what clients Postfix
considers local.

Examples:

The safe setting: append "domain.invalid" to incomplete header addresses
from remote SMTP clients, so that those addresses cannot be confused
with local addresses.

remote_header_rewrite_domain = domain.invalid

The default, purist, setting: don't rewrite headers from remote clients
at all.

remote_header_rewrite_domain =

require_home_directory (default: no)
Require that a local(8) recipient's home directory exists before mail
delivery is attempted. By default this test is disabled. It can be use-
ful for environments that import home directories to the mail server
(IMPORTING HOME DIRECTORIES IS NOT RECOMMENDED).

reset_owner_alias (default: no)
Reset the local(8) delivery agent's idea of the owner-alias attribute,
when delivering mail to a child alias that does not have its own owner
alias.

This feature is available in Postfix 2.8 and later. With older Postfix
releases, the behavior is as if this parameter is set to "yes".

As documented in aliases(5), when an alias name has a companion alias
named owner-name, this will replace the envelope sender address, so that
delivery errors will be reported to the owner alias instead of the
sender. This configuration is recommended for mailing lists.

A less known property of the owner alias is that it also forces the lo-
cal(8) delivery agent to write local and remote addresses from alias ex-
pansion to a new queue file, instead of attempting to deliver mail to
local addresses as soon as they come out of alias expansion.

Writing local addresses from alias expansion to a new queue file allows
for robust handling of temporary delivery errors: errors with one local
member have no effect on deliveries to other members of the list. On
the other hand, delivery to local addresses as soon as they come out of
alias expansion is fragile: a temporary error with one local address
from alias expansion will cause the entire alias to be expanded repeat-
edly until the error goes away, or until the message expires in the
queue. In that case, a problem with one list member results in multiple
message deliveries to other list members.

The default behavior of Postfix 2.8 and later is to keep the owner-alias
attribute of the parent alias, when delivering mail to a child alias
that does not have its own owner alias. Then, local addresses from that
child alias will be written to a new queue file, and a temporary error
with one local address will not affect delivery to other mailing list
members.

Unfortunately, older Postfix releases reset the owner-alias attribute
when delivering mail to a child alias that does not have its own owner
alias. To be precise, this resets only the decision to create a new
queue file, not the decision to override the envelope sender address.
The local(8) delivery agent then attempts to deliver local addresses as
soon as they come out of child alias expansion. If delivery to any ad-
dress from child alias expansion fails with a temporary error condition,
the entire mailing list may be expanded repeatedly until the mail ex-
pires in the queue, resulting in multiple deliveries of the same message
to mailing list members.

resolve_dequoted_address (default: yes)
Resolve a recipient address safely instead of correctly, by looking in-
side quotes.

By default, the Postfix address resolver does not quote the address lo-
calpart as per RFC 822, so that additional @ or % or ! operators remain
visible. This behavior is safe but it is also technically incorrect.

If you specify "resolve_dequoted_address = no", then the Postfix re-
solver will not know about additional @ etc. operators in the address
localpart. This opens opportunities for obscure mail relay attacks with
user@domain@domain addresses when Postfix provides backup MX service for
Sendmail systems.

resolve_null_domain (default: no)
Resolve an address that ends in the "@" null domain as if the local
hostname were specified, instead of rejecting the address as invalid.

This feature is available in Postfix 2.1 and later. Earlier versions
always resolve the null domain as the local hostname.

The Postfix SMTP server uses this feature to reject mail from or to ad-
dresses that end in the "@" null domain, and from addresses that rewrite
into a form that ends in the "@" null domain.

resolve_numeric_domain (default: no)
Resolve "user@ipaddress" as "user@[ipaddress]", instead of rejecting the
address as invalid.

This feature is available in Postfix 2.3 and later.

respectful_logging (default: see 'postconf -d' output)
Avoid logging that implies white is better than black. Instead use 'al-
lowlist', 'denylist', and variations of those words.

This feature is available in Postfix 3.6 and later.

rewrite_service_name (default: rewrite)
The name of the address rewriting service. This service rewrites ad-
dresses to standard form and resolves them to a (delivery method,
next-hop host, recipient) triple.

This feature is available in Postfix 2.0 and later.

sample_directory (default: /etc/postfix)
The name of the directory with example Postfix configuration files.
Starting with Postfix 2.1, these files have been replaced with the post-
conf(5) manual page.

send_cyrus_sasl_authzid (default: no)
When authenticating to a remote SMTP or LMTP server with the default
setting "no", send no SASL authoriZation ID (authzid); send only the
SASL authentiCation ID (authcid) plus the authcid's password.

The non-default setting "yes" enables the behavior of older Postfix ver-
sions. These always send a SASL authzid that is equal to the SASL auth-
cid, but this causes interoperability problems with some SMTP servers.

This feature is available in Postfix 2.4.4 and later.

sender_based_routing (default: no)
This parameter should not be used. It was replaced by sender_depen-
dent_relayhost_maps in Postfix version 2.3.

sender_bcc_maps (default: empty)
Optional BCC (blind carbon-copy) address lookup tables, indexed by enve-
lope sender address. The BCC address (multiple results are not sup-
ported) is added when mail enters from outside of Postfix.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

The table search order is as follows:

• Look up the "user+extension@domain.tld" address including the op-
tional address extension.

• Look up the "user@domain.tld" address without the optional ad-
dress extension.

• Look up the "user+extension" address local part when the sender
domain equals $myorigin, $mydestination, $inet_interfaces or
$proxy_interfaces.

• Look up the "user" address local part when the sender domain
equals $myorigin, $mydestination, $inet_interfaces or $proxy_in-
terfaces.

• Look up the "@domain.tld" part.

Note: with Postfix 2.2 and earlier the sender will be notified when the
BCC address is undeliverable.

Note: automatic BCC recipients are subject to address canonicalization
(add missing domain), canonical_maps, masquerade_domains, and vir-
tual_alias_maps.

Example:

sender_bcc_maps = hash:/etc/postfix/sender_bcc

After a change, run "postmap /etc/postfix/sender_bcc".

This feature is available in Postfix 2.1 and later.

sender_canonical_classes (default: envelope_sender, header_sender)
What addresses are subject to sender_canonical_maps address mapping. By
default, sender_canonical_maps address mapping is applied to envelope
sender addresses, and to header sender addresses.

Specify one or more of: envelope_sender, header_sender

This feature is available in Postfix 2.2 and later.

sender_canonical_maps (default: empty)
Optional address mapping lookup tables for envelope and header sender
addresses. The table format and lookups are documented in canonical(5).

Example: you want to rewrite the SENDER address "user@ugly.example" to
"user@pretty.example", while still being able to send mail to the RECIP-
IENT address "user@ugly.example".

Note: $sender_canonical_maps is processed before $canonical_maps.

Example:

sender_canonical_maps = hash:/etc/postfix/sender_canonical

sender_dependent_default_transport_maps (default: empty)
A sender-dependent override for the global default_transport parameter
setting. The tables are searched by the envelope sender address and @do-
main. A lookup result of DUNNO terminates the search without overriding
the global default_transport parameter setting. This information is
overruled with the transport(5) table.

This setting affects only the default domain address class (recipient
domains that do not match $mydestination, $inet_interfaces, $proxy_in-
terfaces, $virtual_alias_domains, $virtual_mailbox_domains, or $re-
lay_domains):

• In order of decreasing precedence, the delivery transport is
taken from 1) $transport_maps, 2) $sender_dependent_de-
fault_transport_maps or $default_transport.

Note: this overrides default_transport, not transport_maps, and there-
fore the expected syntax is that of default_transport, not the syntax of
transport_maps. Specifically, this does not support the transport_maps
syntax for null transport, null nexthop, or null email addresses.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

For safety reasons, this feature does not allow $number substitutions in
regular expression maps.

This feature is available in Postfix 2.7 and later.

sender_dependent_relayhost_maps (default: empty)
A sender-dependent override for the global relayhost parameter setting.
The tables are searched by the envelope sender address and @domain. A
lookup result of DUNNO terminates the search without overriding the
global relayhost parameter setting (Postfix 2.6 and later).

In order of decreasing precedence:

• For recipient domains in the default domain address class (do-
mains that do not match mydestination, $inet_interfaces,
$proxy_interfaces, $virtual_alias_domains, $virtual_mailbox_do-
mains, $relay_domains), the nexthop destination is taken from 1)
$transport_maps, 2) $sender_dependent_default_transport_maps or
$default_transport, 3) $sender_dependent_relayhost_maps or $re-
layhost or the recipient domain.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

For safety reasons, this feature does not allow $number substitutions in
regular expression maps.

This feature is available in Postfix 2.3 and later.

sendmail_fix_line_endings (default: always)
Controls how the Postfix sendmail command converts email message line
endings from <CR><LF> into UNIX format (<LF>).

always Always convert message lines ending in <CR><LF>. This setting is
the default with Postfix 2.9 and later.

strict Convert message lines ending in <CR><LF> only if the first input
line ends in <CR><LF>. This setting is backwards-compatible with
Postfix 2.8 and earlier.

never Never convert message lines ending in <CR><LF>. This setting ex-
ists for completeness only.

This feature is available in Postfix 2.9 and later.

sendmail_path (default: see postconf -d output)
A Sendmail compatibility feature that specifies the location of the
Postfix sendmail(1) command. This command can be used to submit mail
into the Postfix queue.

service_name (read-only)
The master.cf service name of a Postfix daemon process. This can be used
to distinguish the logging from different services that use the same
program name.

Example master.cf entries:

# Distinguish inbound MTA logging from submission and submissions logging.
smtp inet n - n - - smtpd
submission inet n - n - - smtpd
-o syslog_name=postfix/$service_name
submissions inet n - n - - smtpd
-o syslog_name=postfix/$service_name

# Distinguish outbound MTA logging from inbound relay logging.
smtp unix - - n - - smtp
relay unix - - n - - smtp
-o syslog_name=postfix/$service_name

This feature is available in Postfix 3.3 and later.

service_throttle_time (default: 60s)
How long the Postfix master(8) waits before forking a server that ap-
pears to be malfunctioning.

setgid_group (default: postdrop)
The group ownership of set-gid Postfix commands and of group-writable
Postfix directories. When this parameter value is changed you need to
re-run "postfix set-permissions" (with Postfix version 2.0 and earlier:
"/etc/postfix/post-install set-permissions".

shlib_directory (default: see 'postconf -d' output)
The location of Postfix dynamically-linked libraries (libpostfix-*.so),
and the default location of Postfix database plugins (postfix-*.so) that
have a relative pathname in the dynamicmaps.cf file. The shlib_direc-
tory parameter defaults to "no" when Postfix dynamically-linked li-
braries and database plugins are disabled at compile time, otherwise it
typically defaults to /usr/lib/postfix or /usr/local/lib/postfix.

Notes:

• The directory specified with shlib_directory should contain only
Postfix-related files. Postfix dynamically-linked libraries and
database plugins should not be installed in a "public" system di-
rectory such as /usr/lib or /usr/local/lib. Linking Postfix dy-
namically-linked library files or database plugins into non-Post-
fix programs is not supported. Postfix dynamically-linked li-
braries and database plugins implement a Postfix-internal API
that changes without maintaining compatibility.

• You can change the shlib_directory value after Postfix is built.
However, you may have to run ldconfig or equivalent to prevent
Postfix programs from failing because the libpostfix-*.so files
are not found. No ldconfig command is needed if you keep the
libpostfix-*.so files in the compiled-in default $shlib_directory
location.

This feature is available in Postfix 3.0 and later.

show_user_unknown_table_name (default: yes)
Display the name of the recipient table in the "User unknown" responses.
The extra detail makes troubleshooting easier but also reveals informa-
tion that is nobody else's business.

This feature is available in Postfix 2.0 and later.

showq_service_name (default: showq)
The name of the showq(8) service. This service produces mail queue sta-
tus reports.

This feature is available in Postfix 2.0 and later.

smtp_address_preference (default: any)
The address type ("ipv6", "ipv4" or "any") that the Postfix SMTP client
will try first, when a destination has IPv6 and IPv4 addresses with
equal MX preference. This feature has no effect unless the inet_proto-
cols setting enables both IPv4 and IPv6.

Postfix SMTP client address preference has evolved. With Postfix 2.8 the
default is "ipv6"; earlier implementations are hard-coded to prefer IPv6
over IPv4.

Notes for mail delivery between sites that have both IPv4 and IPv6 con-
nectivity:

• The setting "smtp_address_preference = ipv6" is unsafe. All de-
liveries will suffer delays during an IPv6 outage, even while the
destination is still reachable over IPv4. Mail may be stuck in
the queue with Postfix versions < 3.3 that do not implement
"smtp_balance_inet_protocols". For similar reasons, the setting
"smtp_address_preference = ipv4" is also unsafe.

• The setting "smtp_address_preference = any" is safe. With this,
and "smtp_balance_inet_protocols = yes" (the default), only half
of deliveries will suffer delays if there is an outage that af-
fects IPv6 or IPv4, as long as it does not affect both.

• The setting "smtp_address_preference = ipv4" is not a solution
for remote servers that flag email received over IPv6 as more
'spammy' (the client IPv6 address has a bad or missing PTR or
AAAA record, bad network neighbors, etc.). Instead, configure
Postfix to receive mail over both IPv4 and IPv6, and to deliver
mail over only IPv4.

/etc/postfix/main.cf:
inet_protocols = all

/etc/postfix/master.cf
smtp ...other fields... smtp -o inet_protocols=ipv4

This feature is available in Postfix 2.8 and later.

smtp_address_verify_target (default: rcpt)
In the context of email address verification, the SMTP protocol stage
that determines whether an email address is deliverable. Specify one of
"rcpt" or "data". The latter is needed with remote SMTP servers that
reject recipients after the DATA command. Use transport_maps to apply
this feature selectively:

/etc/postfix/main.cf:
transport_maps = hash:/etc/postfix/transport

/etc/postfix/transport:
smtp-domain-that-verifies-after-data smtp-data-target:
lmtp-domain-that-verifies-after-data lmtp-data-target:

/etc/postfix/master.cf:
smtp-data-target unix - - n - - smtp
-o smtp_address_verify_target=data
lmtp-data-target unix - - n - - lmtp
-o lmtp_address_verify_target=data

Unselective use of the "data" target does no harm, but will result in
unnecessary "lost connection after DATA" events at remote SMTP/LMTP
servers.

This feature is available in Postfix 3.0 and later.

smtp_always_send_ehlo (default: yes)
Always send EHLO at the start of an SMTP session.

With "smtp_always_send_ehlo = no", the Postfix SMTP client sends EHLO
only when the word "ESMTP" appears in the server greeting banner (exam-
ple: 220 spike.porcupine.org ESMTP Postfix).

smtp_balance_inet_protocols (default: yes)
When a remote destination resolves to a combination of IPv4 and IPv6 ad-
dresses, ensure that the Postfix SMTP client can try both address types
before it runs into the smtp_mx_address_limit.

This avoids an interoperability problem when a destination resolves to
primarily IPv6 addresses, the smtp_address_limit feature eliminates most
or all IPv4 addresses, and the destination is not reachable over IPv6.

This feature is available in Postfix 3.3 and later.

smtp_bind_address (default: empty)
An optional numerical network address that the Postfix SMTP client
should bind to when making an IPv4 connection.

This can be specified in the main.cf file for all SMTP clients, or it
can be specified in the master.cf file for a specific client, for exam-
ple:

/etc/postfix/master.cf:
smtp ... smtp -o smtp_bind_address=11.22.33.44

See smtp_bind_address_enforce for how Postfix should handle errors
(Postfix 3.7 and later).

Note 1: when inet_interfaces specifies no more than one IPv4 address,
and that address is a non-loopback address, it is automatically used as
the smtp_bind_address. This supports virtual IP hosting, but can be a
problem on multi-homed firewalls. See the inet_interfaces documentation
for more detail.

Note 2: address information may be enclosed inside [], but this form is
not required here.

smtp_bind_address6 (default: empty)
An optional numerical network address that the Postfix SMTP client
should bind to when making an IPv6 connection.

This feature is available in Postfix 2.2 and later.

This can be specified in the main.cf file for all SMTP clients, or it
can be specified in the master.cf file for a specific client, for exam-
ple:

/etc/postfix/master.cf:
smtp ... smtp -o smtp_bind_address6=1:2:3:4:5:6:7:8

See smtp_bind_address_enforce for how Postfix should handle errors
(Postfix 3.7 and later).

Note 1: when inet_interfaces specifies no more than one IPv6 address,
and that address is a non-loopback address, it is automatically used as
the smtp_bind_address6. This supports virtual IP hosting, but can be a
problem on multi-homed firewalls. See the inet_interfaces documentation
for more detail.

Note 2: address information may be enclosed inside [], but this form is
not recommended here.

smtp_bind_address_enforce (default: no)
Defer delivery when the Postfix SMTP client cannot apply the
smtp_bind_address or smtp_bind_address6 setting. By default, the Postfix
SMTP client will continue delivery after logging a warning.

This feature is available in Postfix 3.7 and later.

smtp_body_checks (default: empty)
Restricted body_checks(5) tables for the Postfix SMTP client. These ta-
bles are searched while mail is being delivered. Actions that change
the delivery time or destination are not available.

This feature is available in Postfix 2.5 and later.

smtp_cname_overrides_servername (default: version dependent)
When the remote SMTP servername is a DNS CNAME, replace the servername
with the result from CNAME expansion for the purpose of logging, SASL
password lookup, TLS policy decisions, or TLS certificate verification.
The value "no" hardens Postfix smtp_tls_per_site hostname-based policies
against false hostname information in DNS CNAME records, and makes SASL
password file lookups more predictable. This is the default setting as
of Postfix 2.3.

When DNS CNAME records are validated with secure DNS lookups
(smtp_dns_support_level = dnssec), they are always allowed to override
the above servername (Postfix 2.11 and later).

This feature is available in Postfix 2.2.9 and later.

smtp_connect_timeout (default: 30s)
The Postfix SMTP client time limit for completing a TCP connection, or
zero (use the operating system built-in time limit).

When no connection can be made within the deadline, the Postfix SMTP
client tries the next address on the mail exchanger list. Specify 0 to
disable the time limit (i.e. use whatever timeout is implemented by the
operating system).

smtp_connection_cache_destinations (default: empty)
Permanently enable SMTP connection caching for the specified destina-
tions. With SMTP connection caching, a connection is not closed immedi-
ately after completion of a mail transaction. Instead, the connection
is kept open for up to $smtp_connection_cache_time_limit seconds. This
allows connections to be reused for other deliveries, and can improve
mail delivery performance.

Specify a comma or white space separated list of destinations or
pseudo-destinations:

• if mail is sent without a relay host: a domain name (the
right-hand side of an email address, without the [] around a nu-
meric IP address),

• if mail is sent via a relay host: a relay host name (without []
or non-default TCP port), as specified in main.cf or in the
transport map,

• if mail is sent via a UNIX-domain socket: a pathname (without the
unix: prefix),

• a /file/name with domain names and/or relay host names as defined
above,

• a "type:table" with domain names and/or relay host names on the
left-hand side. The right-hand side result from "type:table"
lookups is ignored.

This feature is available in Postfix 2.2 and later.

smtp_connection_cache_on_demand (default: yes)
Temporarily enable SMTP connection caching while a destination has a
high volume of mail in the active queue. With SMTP connection caching,
a connection is not closed immediately after completion of a mail trans-
action. Instead, the connection is kept open for up to $smtp_connec-
tion_cache_time_limit seconds. This allows connections to be reused for
other deliveries, and can improve mail delivery performance.

This feature is available in Postfix 2.2 and later.

smtp_connection_cache_time_limit (default: 2s)
When SMTP connection caching is enabled, the amount of time that an un-
used SMTP client socket is kept open before it is closed. Do not spec-
ify larger values without permission from the remote sites.

This feature is available in Postfix 2.2 and later.

smtp_connection_reuse_count_limit (default: 0)
When SMTP connection caching is enabled, the number of times that an
SMTP session may be reused before it is closed, or zero (no limit).
With a reuse count limit of N, a connection is used up to N+1 times.

NOTE: This feature is unsafe. When a high-volume destination has multi-
ple inbound MTAs, then the slowest inbound MTA will attract the most
connections to that destination. This limitation does not exist with
the smtp_connection_reuse_time_limit feature.

This feature is available in Postfix 2.11.

smtp_connection_reuse_time_limit (default: 300s)
The amount of time during which Postfix will use an SMTP connection re-
peatedly. The timer starts when the connection is initiated (i.e. it
includes the connect, greeting and helo latency, in addition to the la-
tencies of subsequent mail delivery transactions).

This feature addresses a performance stability problem with remote SMTP
servers. This problem is not specific to Postfix: it can happen when any
MTA sends large amounts of SMTP email to a site that has multiple MX
hosts.

The problem starts when one of a set of MX hosts becomes slower than the
rest. Even though SMTP clients connect to fast and slow MX hosts with
equal probability, the slow MX host ends up with more simultaneous in-
bound connections than the faster MX hosts, because the slow MX host
needs more time to serve each client request.

The slow MX host becomes a connection attractor. If one MX host becomes
N times slower than the rest, it dominates mail delivery latency unless
there are more than N fast MX hosts to counter the effect. And if the
number of MX hosts is smaller than N, the mail delivery latency becomes
effectively that of the slowest MX host divided by the total number of
MX hosts.

The solution uses connection caching in a way that differs from Postfix
version 2.2. By limiting the amount of time during which a connection
can be used repeatedly (instead of limiting the number of deliveries
over that connection), Postfix not only restores fairness in the distri-
bution of simultaneous connections across a set of MX hosts, it also fa-
vors deliveries over connections that perform well, which is exactly
what we want.

The default reuse time limit, 300s, is comparable to the various smtp
transaction timeouts which are fair estimates of maximum excess latency
for a slow delivery. Note that hosts may accept thousands of messages
over a single connection within the default connection reuse time limit.
This number is much larger than the default Postfix version 2.2 limit of
10 messages per cached connection. It may prove necessary to lower the
limit to avoid interoperability issues with MTAs that exhibit bugs when
many messages are delivered via a single connection. A lower reuse time
limit risks losing the benefit of connection reuse when the average con-
nection and mail delivery latency exceeds the reuse time limit.

This feature is available in Postfix 2.3 and later.

smtp_data_done_timeout (default: 600s)
The Postfix SMTP client time limit for sending the SMTP ".", and for re-
ceiving the remote SMTP server response.

When no response is received within the deadline, a warning is logged
that the mail may be delivered multiple times.

smtp_data_init_timeout (default: 120s)
The Postfix SMTP client time limit for sending the SMTP DATA command,
and for receiving the remote SMTP server response.

Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
The default time unit is s (seconds).

smtp_data_xfer_timeout (default: 180s)
The Postfix SMTP client time limit for sending the SMTP message content.
When the connection makes no progress for more than
$smtp_data_xfer_timeout seconds the Postfix SMTP client terminates the
transfer.

Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
The default time unit is s (seconds).

smtp_defer_if_no_mx_address_found (default: no)
Defer mail delivery when no MX record resolves to an IP address.

The default (no) is to return the mail as undeliverable. With older
Postfix versions the default was to keep trying to deliver the mail un-
til someone fixed the MX record or until the mail was too old.

Note: the Postfix SMTP client always ignores MX records with equal or
worse preference than the local MTA itself.

This feature is available in Postfix 2.1 and later.

smtp_delivery_status_filter (default: $default_delivery_status_filter)
Optional filter for the smtp(8) delivery agent to change the delivery
status code or explanatory text of successful or unsuccessful deliver-
ies. See default_delivery_status_filter for details.

NOTE: This feature modifies Postfix SMTP client error or non-error mes-
sages that may or may not be derived from remote SMTP server responses.
In contrast, the smtp_reply_filter feature modifies remote SMTP server
responses only.

smtp_destination_concurrency_limit (default: $default_destination_concur-
rency_limit)
The maximal number of parallel deliveries to the same destination via
the smtp message delivery transport. This limit is enforced by the queue
manager. The message delivery transport name is the first field in the
entry in the master.cf file.

smtp_destination_recipient_limit (default: $default_destination_recipi-
ent_limit)
The maximal number of recipients per message for the smtp message deliv-
ery transport. This limit is enforced by the queue manager. The message
delivery transport name is the first field in the entry in the master.cf
file.

Setting this parameter to a value of 1 changes the meaning of smtp_des-
tination_concurrency_limit from concurrency per domain into concurrency
per recipient.

smtp_discard_ehlo_keyword_address_maps (default: empty)
Lookup tables, indexed by the remote SMTP server address, with case in-
sensitive lists of EHLO keywords (pipelining, starttls, auth, etc.) that
the Postfix SMTP client will ignore in the EHLO response from a remote
SMTP server. See smtp_discard_ehlo_keywords for details. The table is
not indexed by hostname for consistency with smtpd_discard_ehlo_key-
word_address_maps.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

This feature is available in Postfix 2.2 and later.

smtp_discard_ehlo_keywords (default: empty)
A case insensitive list of EHLO keywords (pipelining, starttls, auth,
etc.) that the Postfix SMTP client will ignore in the EHLO response from
a remote SMTP server.

This feature is available in Postfix 2.2 and later.

Notes:

• Specify the silent-discard pseudo keyword to prevent this action
from being logged.

• Use the smtp_discard_ehlo_keyword_address_maps feature to discard
EHLO keywords selectively.

smtp_dns_reply_filter (default: empty)
Optional filter for Postfix SMTP client DNS lookup results. Specify
zero or more lookup tables. The lookup tables are searched in the given
order for a match with the DNS lookup result, converted to the following
form:

name ttl class type preference value

The class field is always "IN", the preference field exists only for MX
records, the names of hosts, domains, etc. end in ".", and those names
are in ASCII form (xn--mumble form in the case of UTF8 names).

When a match is found, the table lookup result specifies an action. By
default, the table query and the action name are case-insensitive. Cur-
rently, only the IGNORE action is implemented.

Notes:

• Postfix DNS reply filters have no effect on implicit DNS lookups
through nsswitch.conf or equivalent mechanisms.

• The Postfix SMTP/LMTP client uses smtp_dns_reply_filter and
lmtp_dns_reply_filter only to discover a remote SMTP or LMTP ser-
vice (record types MX, A, AAAA, and TLSA). These lookups are
also made to implement the features reject_unverified_sender and
reject_unverified_recipient.

• The Postfix SMTP/LMTP client defers mail delivery when a filter
removes all lookup results from a successful query.

• Postfix SMTP server uses smtpd_dns_reply_filter only to look up
MX, A, AAAA, and TXT records to implement the features reject_un-
known_helo_hostname, reject_unknown_sender_domain, reject_un-
known_recipient_domain, reject_rbl_*, and reject_rhsbl_*.

• The Postfix SMTP server logs a warning or defers mail delivery
when a filter removes all lookup results from a successful query.

Example: ignore Google AAAA records in Postfix SMTP client DNS lookups,
because Google sometimes hard-rejects mail from IPv6 clients with valid
PTR etc. records.

/etc/postfix/main.cf:
smtp_dns_reply_filter = pcre:/etc/postfix/smtp_dns_reply_filter

/etc/postfix/smtp_dns_reply_filter:
# /domain ttl IN AAAA address/ action, all case-insensitive.
# Note: the domain name ends in ".".
/^\S+\.google\.com\.\s+\S+\s+\S+\s+AAAA\s+/ IGNORE

This feature is available in Postfix 3.0 and later.

smtp_dns_resolver_options (default: empty)
DNS Resolver options for the Postfix SMTP client. Specify zero or more
of the following options, separated by comma or whitespace. Option
names are case-sensitive. Some options refer to domain names that are
specified in the file /etc/resolv.conf or equivalent.

res_defnames
Append the current domain name to single-component names (those
that do not contain a "." character). This can produce incorrect
results, and is the hard-coded behavior prior to Postfix 2.8.

res_dnsrch
Search for host names in the current domain and in parent do-
mains. This can produce incorrect results and is therefore not
recommended.

This feature is available in Postfix 2.8 and later.

smtp_dns_support_level (default: empty)
Level of DNS support in the Postfix SMTP client. With "smtp_dns_sup-
port_level" left at its empty default value, the legacy "dis-
able_dns_lookups" parameter controls whether DNS is enabled in the Post-
fix SMTP client, otherwise the legacy parameter is ignored.

Specify one of the following:

disabled
Disable DNS lookups. No MX lookups are performed and hostname to
address lookups are unconditionally "native". This setting is
not appropriate for hosts that deliver mail to the public Inter-
net. Some obsolete how-to documents recommend disabling DNS
lookups in some configurations with content_filters. This is no
longer required and strongly discouraged.

enabled
Enable DNS lookups. Nexthop destination domains not enclosed in
"[]" will be subject to MX lookups. If "dns" and "native" are
included in the "smtp_host_lookup" parameter value, DNS will be
queried first to resolve MX-host A records, followed by "native"
lookups if no answer is found in DNS.

dnssec Enable DNSSEC lookups. The "dnssec" setting differs from the
"enabled" setting above in the following ways:

• Any MX lookups will set RES_USE_DNSSEC and RES_USE_EDNS0 to re-
quest DNSSEC-validated responses. If the MX response is
DNSSEC-validated the corresponding hostnames are considered vali-
dated.

• The address lookups of validated hostnames are also validated,
(provided of course "smtp_host_lookup" includes "dns", see be-
low).

• Temporary failures in DNSSEC-enabled hostname-to-address resolu-
tion block any "native" lookups. Additional "native" lookups
only happen when DNSSEC lookups hard-fail (NODATA or NXDOMAIN).

The Postfix SMTP client considers non-MX "[nexthop]" and "[nex-
thop]:port" destinations equivalent to statically-validated MX records
of the form "nexthop. IN MX 0 nexthop." Therefore, with "dnssec" sup-
port turned on, validated hostname-to-address lookups apply to the nex-
thop domain of any "[nexthop]" or "[nexthop]:port" destination. This is
also true for LMTP "inet:host" and "inet:host:port" destinations, as
LMTP hostnames are never subject to MX lookups.

The "dnssec" setting is recommended only if you plan to use the dane or
dane-only TLS security level, otherwise enabling DNSSEC support in Post-
fix offers no additional security. Postfix DNSSEC support relies on an
upstream recursive nameserver that validates DNSSEC signatures. Such a
DNS server will always filter out forged DNS responses, even when Post-
fix itself is not configured to use DNSSEC.

When using Postfix DANE support the "smtp_host_lookup" parameter should
include "dns", as DANE is not applicable to hosts resolved via "native"
lookups.

As mentioned above, Postfix is not a validating stub resolver; it relies
on the system's configured DNSSEC-validating recursive nameserver to
perform all DNSSEC validation. Since this nameserver's DNSSEC-validated
responses will be fully trusted, it is strongly recommended that the MTA
host have a local DNSSEC-validating recursive caching nameserver listen-
ing on a loopback address, and be configured to use only this nameserver
for all lookups. Otherwise, Postfix may remain subject to
man-in-the-middle attacks that forge responses from the recursive name-
server

DNSSEC support requires a version of Postfix compiled against a reason-
ably-modern DNS resolver(3) library that implements the RES_USE_DNSSEC
and RES_USE_EDNS0 resolver options.

This feature is available in Postfix 2.11 and later.

smtp_enforce_tls (default: no)
Enforcement mode: require that remote SMTP servers use TLS encryption,
and never send mail in the clear. This also requires that the remote
SMTP server hostname matches the information in the remote server cer-
tificate, and that the remote SMTP server certificate was issued by a CA
that is trusted by the Postfix SMTP client. If the certificate doesn't
verify or the hostname doesn't match, delivery is deferred and mail
stays in the queue.

The server hostname is matched against all names provided as dNSNames in
the SubjectAlternativeName. If no dNSNames are specified, the Common-
Name is checked. The behavior may be changed with the smtp_tls_en-
force_peername option.

This option is useful only if you are definitely sure that you will only
connect to servers that support RFC 2487 _and_ that provide valid server
certificates. Typical use is for clients that send all their email to a
dedicated mailhub.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and
later use smtp_tls_security_level instead.

smtp_fallback_relay (default: $fallback_relay)
Optional list of relay destinations that will be used when an SMTP des-
tination is not found, or when delivery fails due to a non-permanent er-
ror. With Postfix 2.2 and earlier this parameter is called fallback_re-
lay.

By default, smtp_fallback_relay is empty, mail is returned to the sender
when a destination is not found, and delivery is deferred after it fails
due to a non-permanent error.

With bulk email deliveries, it can be beneficial to run the fallback re-
lay MTA on the same host, so that it can reuse the sender IP address.
This speeds up deliveries that are delayed by IP-based reputation sys-
tems (greylist, etc.).

To prevent mailer loops between MX hosts and fall-back hosts, Postfix
version 2.2 and later will not use the fallback relays for destinations
that it is MX host for (assuming DNS lookup is turned on).

smtp_generic_maps (default: empty)
Optional lookup tables that perform address rewriting in the Postfix
SMTP client, typically to transform a locally valid address into a glob-
ally valid address when sending mail across the Internet. This is
needed when the local machine does not have its own Internet domain
name, but uses something like localdomain.local instead.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

The table format and lookups are documented in generic(5); examples are
shown in the ADDRESS_REWRITING_README and STANDARD_CONFIGURATION_README
documents.

This feature is available in Postfix 2.2 and later.

smtp_header_checks (default: empty)
Restricted header_checks(5) tables for the Postfix SMTP client. These
tables are searched while mail is being delivered. Actions that change
the delivery time or destination are not available.

This feature is available in Postfix 2.5 and later.

smtp_helo_name (default: $myhostname)
The hostname to send in the SMTP HELO or EHLO command.

The default value is the machine hostname. Specify a hostname or
[ip.add.re.ss].

This information can be specified in the main.cf file for all SMTP
clients, or it can be specified in the master.cf file for a specific
client, for example:

/etc/postfix/master.cf:
mysmtp ... smtp -o smtp_helo_name=foo.bar.com

This feature is available in Postfix 2.0 and later.

smtp_helo_timeout (default: 300s)
The Postfix SMTP client time limit for sending the HELO or EHLO command,
and for receiving the initial remote SMTP server response.

smtp_host_lookup (default: dns)
What mechanisms the Postfix SMTP client uses to look up a host's IP ad-
dress. This parameter is ignored when DNS lookups are disabled (see:
disable_dns_lookups and smtp_dns_support_level). The "dns" mechanism is
always tried before "native" if both are listed.

Specify one of the following:

dns Hosts can be found in the DNS (preferred).

native Use the native naming service only (nsswitch.conf, or equivalent
mechanism).

dns, native
Use the native service for hosts not found in the DNS.

This feature is available in Postfix 2.1 and later.

smtp_line_length_limit (default: 998)
The maximal length of message header and body lines that Postfix will
send via SMTP. This limit does not include the <CR><LF> at the end of
each line. Longer lines are broken by inserting "<CR><LF><SPACE>", to
minimize the damage to MIME formatted mail. Specify zero to disable this
limit.

The Postfix limit of 998 characters not including <CR><LF> is consistent
with the SMTP limit of 1000 characters including <CR><LF>. The Postfix
limit was 990 with Postfix 2.8 and earlier.

smtp_mail_timeout (default: 300s)
The Postfix SMTP client time limit for sending the MAIL FROM command,
and for receiving the remote SMTP server response.

smtp_mime_header_checks (default: empty)
Restricted mime_header_checks(5) tables for the Postfix SMTP client.
These tables are searched while mail is being delivered. Actions that
change the delivery time or destination are not available.

This feature is available in Postfix 2.5 and later.

smtp_min_data_rate (default: 500)
The minimum plaintext data transfer rate in bytes/second for DATA re-
quests, when deadlines are enabled with smtp_per_request_deadline. Af-
ter a write operation transfers N plaintext message bytes (possibly af-
ter TLS encryption), and after the DATA request deadline is decremented
by the elapsed time of that write operation, the DATA request deadline
is incremented by N/smtp_min_data_rate seconds. However, the deadline
will never be incremented beyond the time limit specified with
smtp_data_xfer_timeout.

This feature is available in Postfix 3.7 and later.

smtp_mx_address_limit (default: 5)
The maximal number of MX (mail exchanger) IP addresses that can result
from Postfix SMTP client mail exchanger lookups, or zero (no limit).
Prior to Postfix version 2.3, this limit was disabled by default.

This feature is available in Postfix 2.1 and later.

smtp_mx_session_limit (default: 2)
The maximal number of SMTP sessions per delivery request before the
Postfix SMTP client gives up or delivers to a fall-back relay host, or
zero (no limit). This restriction ignores sessions that fail to complete
the SMTP initial handshake (Postfix version 2.2 and earlier) or that
fail to complete the EHLO and TLS handshake (Postfix version 2.3 and
later).

This feature is available in Postfix 2.1 and later.

smtp_nested_header_checks (default: empty)
Restricted nested_header_checks(5) tables for the Postfix SMTP client.
These tables are searched while mail is being delivered. Actions that
change the delivery time or destination are not available.

This feature is available in Postfix 2.5 and later.

smtp_never_send_ehlo (default: no)
Never send EHLO at the start of an SMTP session. See also the smtp_al-
ways_send_ehlo parameter.

smtp_per_record_deadline (default: no)
Change the behavior of the smtp_*_timeout time limits, from a time limit
per read or write system call, to a time limit to send or receive a com-
plete record (an SMTP command line, SMTP response line, SMTP message
content line, or TLS protocol message). This limits the impact from
hostile peers that trickle data one byte at a time.

Note: when per-record deadlines are enabled, a short timeout may cause
problems with TLS over very slow network connections. The reasons are
that a TLS protocol message can be up to 16 kbytes long (with TLSv1),
and that an entire TLS protocol message must be sent or received within
the per-record deadline.

This feature is available in Postfix 2.9-3.6. With older Postfix re-
leases, the behavior is as if this parameter is set to "no". Postfix 3.7
and later use smtp_per_request_deadline.

smtp_per_request_deadline (default: no)
Change the behavior of the smtp_*_timeout time limits, from a time limit
per plaintext or TLS read or write call, to a combined time limit for
sending a complete SMTP request and for receiving a complete SMTP re-
sponse. The deadline limits only the time spent waiting for plaintext or
TLS read or write calls, not time spent elsewhere. The per-request dead-
line limits the impact from hostile peers that trickle data one byte at
a time.

See smtp_min_data_rate for how the per-request deadline is managed dur-
ing the DATA phase.

Note: when per-request deadlines are enabled, a short time limit may
cause problems with TLS over very slow network connections. The reason
is that a TLS protocol message can be up to 16 kbytes long (with TLSv1),
and that an entire TLS protocol message must be transferred within the
per-request deadline.

This feature is available in Postfix 3.7 and later. A weaker feature,
called smtp_per_record_deadline, is available with Postfix 2.9-3.6.

This feature is available in Postfix 3.7 and later.

smtp_pix_workaround_delay_time (default: 10s)
How long the Postfix SMTP client pauses before sending ".<CR><LF>" in
order to work around the PIX firewall "<CR><LF>.<CR><LF>" bug.

Choosing too short a time makes this workaround ineffective when sending
large messages over slow network connections.

smtp_pix_workaround_maps (default: empty)
Lookup tables, indexed by the remote SMTP server address, with per-des-
tination workarounds for CISCO PIX firewall bugs. The table is not in-
dexed by hostname for consistency with smtp_discard_ehlo_keyword_ad-
dress_maps.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

This feature is available in Postfix 2.4 and later.

smtp_pix_workaround_threshold_time (default: 500s)
How long a message must be queued before the Postfix SMTP client turns
on the PIX firewall "<CR><LF>.<CR><LF>" bug workaround for delivery
through firewalls with "smtp fixup" mode turned on.

By default, the workaround is turned off for mail that is queued for
less than 500 seconds. In other words, the workaround is normally turned
off for the first delivery attempt.

Specify 0 to enable the PIX firewall "<CR><LF>.<CR><LF>" bug workaround
upon the first delivery attempt.

smtp_pix_workarounds (default: disable_esmtp, delay_dotcrlf)
A list that specifies zero or more workarounds for CISCO PIX firewall
bugs. These workarounds are implemented by the Postfix SMTP client.
Workaround names are separated by comma or space, and are case insensi-
tive. This parameter setting can be overruled with per-destination
smtp_pix_workaround_maps settings.

delay_dotcrlf
Insert a delay before sending ".<CR><LF>" after the end of the
message content. The delay is subject to the
smtp_pix_workaround_delay_time and smtp_pix_workaround_thresh-
old_time parameter settings.

disable_esmtp
Disable all extended SMTP commands: send HELO instead of EHLO.

This feature is available in Postfix 2.4 and later. The default settings
are backwards compatible with earlier Postfix versions.

smtp_quit_timeout (default: 300s)
The Postfix SMTP client time limit for sending the QUIT command, and for
receiving the remote SMTP server response.

smtp_quote_rfc821_envelope (default: yes)
Quote addresses in Postfix SMTP client MAIL FROM and RCPT TO commands as
required by RFC 5321. This includes putting quotes around an address lo-
calpart that ends in ".".

The default is to comply with RFC 5321. If you have to send mail to a
broken SMTP server, configure a special SMTP client in master.cf:

/etc/postfix/master.cf:
broken-smtp . . . smtp -o smtp_quote_rfc821_envelope=no

and route mail for the destination in question to the "broken-smtp" mes-
sage delivery with a transport(5) table.

This feature is available in Postfix 2.1 and later.

smtp_randomize_addresses (default: yes)
Randomize the order of equal-preference MX host addresses. This is a
performance feature of the Postfix SMTP client.

smtp_rcpt_timeout (default: 300s)
The Postfix SMTP client time limit for sending the SMTP RCPT TO command,
and for receiving the remote SMTP server response.

smtp_reply_filter (default: empty)
A mechanism to transform replies from remote SMTP servers one line at a
time. This is a last-resort tool to work around server replies that
break interoperability with the Postfix SMTP client. Other uses involve
fault injection to test Postfix's handling of invalid responses.

Notes:

• In the case of a multi-line reply, the Postfix SMTP client uses
the final reply line's numerical SMTP reply code and enhanced
status code.

• The numerical SMTP reply code (XYZ) takes precedence over the en-
hanced status code (X.Y.Z). When the enhanced status code ini-
tial digit differs from the SMTP reply code initial digit, or
when no enhanced status code is present, the Postfix SMTP client
uses a generic enhanced status code (X.0.0) instead.

Specify the name of a "type:table" lookup table. The search string is a
single SMTP reply line as received from the remote SMTP server, except
that the trailing <CR><LF> are removed. When the lookup succeeds, the
result replaces the single SMTP reply line.

Examples:

/etc/postfix/main.cf:
smtp_reply_filter = pcre:/etc/postfix/reply_filter

/etc/postfix/reply_filter:
# Transform garbage into "250-filler..." so that it looks like
# one line from a multi-line reply. It does not matter what we
# substitute here as long it has the right syntax. The Postfix
# SMTP client will use the final line's numerical SMTP reply
# code and enhanced status code.
!/^([2-5][0-9][0-9]($|[- ]))/ 250-filler for garbage

This feature is available in Postfix 2.7.

smtp_rset_timeout (default: 20s)
The Postfix SMTP client time limit for sending the RSET command, and for
receiving the remote SMTP server response. The SMTP client sends RSET in
order to finish a recipient address probe, or to verify that a cached
session is still usable.

This feature is available in Postfix 2.1 and later.

smtp_sasl_auth_cache_name (default: empty)
An optional table to prevent repeated SASL authentication failures with
the same remote SMTP server hostname, username and password. Each table
(key, value) pair contains a server name, a username and password, and
the full server response. This information is stored when a remote SMTP
server rejects an authentication attempt with a 535 reply code. As long
as the smtp_sasl_password_maps information does not change, and as long
as the smtp_sasl_auth_cache_name information does not expire (see
smtp_sasl_auth_cache_time) the Postfix SMTP client avoids SASL authenti-
cation attempts with the same server, username and password, and instead
bounces or defers mail as controlled with the smtp_sasl_auth_soft_bounce
configuration parameter.

Use a per-destination delivery concurrency of 1 (for example, "smtp_des-
tination_concurrency_limit = 1", "relay_destination_concurrency_limit =
1", etc.), otherwise multiple delivery agents may experience a login
failure at the same time.

The table must be accessed via the proxywrite service, i.e. the map name
must start with "proxy:". The table should be stored under the directory
specified with the data_directory parameter.

This feature uses cryptographic hashing to protect plain-text passwords,
and requires that Postfix is compiled with TLS support.

Example:

smtp_sasl_auth_cache_name = proxy:btree:/var/lib/postfix/sasl_auth_cache

This feature is available in Postfix 2.5 and later.

smtp_sasl_auth_cache_time (default: 90d)
The maximal age of an smtp_sasl_auth_cache_name entry before it is re-
moved.

This feature is available in Postfix 2.5 and later.

smtp_sasl_auth_enable (default: no)
Enable SASL authentication in the Postfix SMTP client. By default, the
Postfix SMTP client uses no authentication.

Example:

smtp_sasl_auth_enable = yes

smtp_sasl_auth_soft_bounce (default: yes)
When a remote SMTP server rejects a SASL authentication request with a
535 reply code, defer mail delivery instead of returning mail as unde-
liverable. The latter behavior was hard-coded prior to Postfix version
2.5.

Note: the setting "yes" overrides the global soft_bounce parameter, but
the setting "no" does not.

Example:

# Default as of Postfix 2.5
smtp_sasl_auth_soft_bounce = yes
# The old hard-coded default
smtp_sasl_auth_soft_bounce = no

This feature is available in Postfix 2.5 and later.

smtp_sasl_mechanism_filter (default: empty)
If non-empty, a Postfix SMTP client filter for the remote SMTP server's
list of offered SASL mechanisms. Different client and server implemen-
tations may support different mechanism lists; by default, the Postfix
SMTP client will use the intersection of the two. smtp_sasl_mecha-
nism_filter specifies an optional third mechanism list to intersect
with.

Specify mechanism names, "/file/name" patterns or "type:table" lookup
tables. The right-hand side result from "type:table" lookups is ignored.
Specify "!pattern" to exclude a mechanism name from the list. The form
"!/file/name" is supported only in Postfix version 2.4 and later.

This feature is available in Postfix 2.2 and later.

Examples:

smtp_sasl_mechanism_filter = plain, login
smtp_sasl_mechanism_filter = /etc/postfix/smtp_mechs
smtp_sasl_mechanism_filter = !gssapi, !login, static:rest

smtp_sasl_password_maps (default: empty)
Optional Postfix SMTP client lookup tables with one username:password
entry per sender, remote hostname or next-hop domain. Per-sender lookup
is done only when sender-dependent authentication is enabled. If no
username:password entry is found, then the Postfix SMTP client will not
attempt to authenticate to the remote host.

Use smtp_sasl_password_result_delimiter to specify an alternative sepa-
rator between username and password.

The Postfix SMTP client opens the lookup table before going to chroot
jail, so you can leave the password file in /etc/postfix.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

smtp_sasl_password_result_delimiter (default: :)
The delimiter between username and password in sasl_passwd_maps lookup
results. Specify one non-whitespace character that does not appear in
the username.

This feature is available in Postfix >= 3.9.

smtp_sasl_path (default: empty)
Implementation-specific information that the Postfix SMTP client passes
through to the SASL plug-in implementation that is selected with
smtp_sasl_type. Typically this specifies the name of a configuration
file or rendezvous point.

This feature is available in Postfix 2.3 and later.

smtp_sasl_security_options (default: noplaintext, noanonymous)
Postfix SMTP client SASL security options; as of Postfix 2.3 the list of
available features depends on the SASL client implementation that is se-
lected with smtp_sasl_type.

The following security features are defined for the cyrus client SASL
implementation:

Specify zero or more of the following:

noplaintext
Disallow methods that use plaintext passwords.

noactive
Disallow methods subject to active (non-dictionary) attack.

nodictionary
Disallow methods subject to passive (dictionary) attack.

noanonymous
Disallow methods that allow anonymous authentication.

mutual_auth
Only allow methods that provide mutual authentication (not avail-
able with SASL version 1).

Example:

smtp_sasl_security_options = noplaintext

smtp_sasl_tls_security_options (default: $smtp_sasl_security_options)
The SASL authentication security options that the Postfix SMTP client
uses for TLS encrypted SMTP sessions.

This feature is available in Postfix 2.2 and later.

smtp_sasl_tls_verified_security_options (default: $smtp_sasl_tls_security_op-
tions)
The SASL authentication security options that the Postfix SMTP client
uses for TLS encrypted SMTP sessions with a verified server certificate.

When mail is sent to the public MX host for the recipient's domain,
server certificates are by default optional, and delivery proceeds even
if certificate verification fails. For delivery via a submission service
that requires SASL authentication, it may be appropriate to send plain-
text passwords only when the connection to the server is strongly en-
crypted and the server identity is verified.

The smtp_sasl_tls_verified_security_options parameter makes it possible
to only enable plaintext mechanisms when a secure connection to the
server is available. Submission servers subject to this policy must ei-
ther have verifiable certificates or offer suitable non-plaintext SASL
mechanisms.

This feature is available in Postfix 2.6 and later.

smtp_sasl_type (default: cyrus)
The SASL plug-in type that the Postfix SMTP client should use for au-
thentication. The available types are listed with the "postconf -A"
command.

This feature is available in Postfix 2.3 and later.

smtp_send_dummy_mail_auth (default: no)
Whether or not to append the "AUTH=<>" option to the MAIL FROM command
in SASL-authenticated SMTP sessions. The default is not to send this, to
avoid problems with broken remote SMTP servers. Before Postfix 2.9 the
behavior is as if "smtp_send_dummy_mail_auth = yes".

This feature is available in Postfix 2.9 and later.

smtp_send_xforward_command (default: no)
Send the non-standard XFORWARD command when the Postfix SMTP server EHLO
response announces XFORWARD support.

This allows a Postfix SMTP delivery agent, used for injecting mail into
a content filter, to forward the name, address, protocol and HELO name
of the original client to the content filter and downstream queuing SMTP
server. This can produce more useful logging than localhost[127.0.0.1]
etc.

This feature is available in Postfix 2.1 and later.

smtp_sender_dependent_authentication (default: no)
Enable sender-dependent authentication in the Postfix SMTP client; this
is available only with SASL authentication, and disables SMTP connection
caching to ensure that mail from different senders will use the appro-
priate credentials.

This feature is available in Postfix 2.3 and later.

smtp_skip_4xx_greeting (default: yes)
Skip SMTP servers that greet with a 4XX status code (go away, try again
later).

By default, the Postfix SMTP client moves on the next mail exchanger.
Specify "smtp_skip_4xx_greeting = no" if Postfix should defer delivery
immediately.

This feature is available in Postfix 2.0 and earlier. Later Postfix
versions always skip remote SMTP servers that greet with a 4XX status
code.

smtp_skip_5xx_greeting (default: yes)
Skip remote SMTP servers that greet with a 5XX status code.

By default, the Postfix SMTP client moves on the next mail exchanger.
Specify "smtp_skip_5xx_greeting = no" if Postfix should bounce the mail
immediately. Caution: the latter behavior appears to contradict RFC
2821.

smtp_skip_quit_response (default: yes)
Do not wait for the response to the SMTP QUIT command.

smtp_starttls_timeout (default: 300s)
Time limit for Postfix SMTP client write and read operations during TLS
startup and shutdown handshake procedures.

This feature is available in Postfix 2.2 and later.

smtp_tcp_port (default: smtp)
The default TCP port that the Postfix SMTP client connects to. Specify
a symbolic name (see services(5)) or a numeric port.

smtp_tls_CAfile (default: empty)
A file containing CA certificates of root CAs trusted to sign either re-
mote SMTP server certificates or intermediate CA certificates. These
are loaded into memory before the smtp(8) client enters the chroot jail.
If the number of trusted roots is large, consider using smtp_tls_CApath
instead, but note that the latter directory must be present in the ch-
root jail if the smtp(8) client is chrooted. This file may also be used
to augment the client certificate trust chain, but it is best to include
all the required certificates directly in $smtp_tls_cert_file (or, Post-
fix >= 3.4 $smtp_tls_chain_files).

Specify "smtp_tls_CAfile = /path/to/system_CA_file" to use ONLY the sys-
tem-supplied default Certification Authority certificates.

Specify "tls_append_default_CA = no" to prevent Postfix from appending
the system-supplied default CAs and trusting third-party certificates.

Example:

smtp_tls_CAfile = /etc/postfix/CAcert.pem

This feature is available in Postfix 2.2 and later.

smtp_tls_CApath (default: empty)
Directory with PEM format Certification Authority certificates that the
Postfix SMTP client uses to verify a remote SMTP server certificate.
Don't forget to create the necessary "hash" links with, for example,
"$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs".

To use this option in chroot mode, this directory (or a copy) must be
inside the chroot jail.

Specify "smtp_tls_CApath = /path/to/system_CA_directory" to use ONLY the
system-supplied default Certification Authority certificates.

Specify "tls_append_default_CA = no" to prevent Postfix from appending
the system-supplied default CAs and trusting third-party certificates.

Example:

smtp_tls_CApath = /etc/postfix/certs

This feature is available in Postfix 2.2 and later.

smtp_tls_block_early_mail_reply (default: no)
Try to detect a mail hijacking attack based on a TLS protocol vulnera-
bility (CVE-2009-3555), where an attacker prepends malicious HELO, MAIL,
RCPT, DATA commands to a Postfix SMTP client TLS session. The attack
would succeed with non-Postfix SMTP servers that reply to the malicious
HELO, MAIL, RCPT, DATA commands after negotiating the Postfix SMTP
client TLS session.

This feature is available in Postfix 2.7.

smtp_tls_cert_file (default: empty)
File with the Postfix SMTP client RSA certificate in PEM format. This
file may also contain the Postfix SMTP client private RSA key, and these
may be the same as the Postfix SMTP server RSA certificate and key file.
With Postfix >= 3.4 the preferred way to configure client keys and cer-
tificates is via the "smtp_tls_chain_files" parameter.

Do not configure client certificates unless you must present client TLS
certificates to one or more servers. Client certificates are not usually
needed, and can cause problems in configurations that work well without
them. The recommended setting is to let the defaults stand:

smtp_tls_cert_file =
smtp_tls_key_file =
smtp_tls_eccert_file =
smtp_tls_eckey_file =
# Obsolete DSA parameters
smtp_tls_dcert_file =
smtp_tls_dkey_file =
# Postfix >= 3.4 interface
smtp_tls_chain_files =

The best way to use the default settings is to comment out the above pa-
rameters in main.cf if present.

To enable remote SMTP servers to verify the Postfix SMTP client certifi-
cate, the issuing CA certificates must be made available to the server.
You should include the required certificates in the client certificate
file, the client certificate first, then the issuing CA(s) (bottom-up
order).

Example: the certificate for "client.example.com" was issued by "inter-
mediate CA" which itself has a certificate issued by "root CA". As the
"root" super-user create the client.pem file with:

# umask 077
# cat client_key.pem client_cert.pem intermediate_CA.pem > chain.pem

If you also want to verify remote SMTP server certificates issued by
these CAs, you can add the CA certificates to the smtp_tls_CAfile, in
which case it is not necessary to have them in the smtp_tls_cert_file,
smtp_tls_dcert_file (obsolete) or smtp_tls_eccert_file.

A certificate supplied here must be usable as an SSL client certificate
and hence pass the "openssl verify -purpose sslclient ..." test.

Example:

smtp_tls_cert_file = /etc/postfix/chain.pem

This feature is available in Postfix 2.2 and later.

smtp_tls_chain_files (default: empty)
List of one or more PEM files, each holding one or more private keys di-
rectly followed by a corresponding certificate chain. The file names
are separated by commas and/or whitespace. This parameter obsoletes the
legacy algorithm-specific key and certificate file settings. When this
parameter is non-empty, the legacy parameters are ignored, and a warning
is logged if any are also non-empty.

With the proliferation of multiple private key algorithms-which, as of
OpenSSL 1.1.1, include DSA (obsolete), RSA, ECDSA, Ed25519 and Ed448-it
is increasingly impractical to use separate parameters to configure the
key and certificate chain for each algorithm. Therefore, Postfix now
supports storing multiple keys and corresponding certificate chains in a
single file or in a set of files.

Each key must appear immediately before the corresponding certificate,
optionally followed by additional issuer certificates that complete the
certificate chain for that key. When multiple files are specified, they
are equivalent to a single file that is concatenated from those files in
the given order. Thus, while a key must always precede its certificate
and issuer chain, it can be in a separate file, so long as that file is
listed immediately before the file that holds the corresponding certifi-
cate chain. Once all the files are concatenated, the sequence of PEM
objects must be: key1, cert1, [chain1], key2, cert2, [chain2], ...,
keyN, certN, [chainN].

Storing the private key in the same file as the corresponding certifi-
cate is more reliable. With the key and certificate in separate files,
there is a chance that during key rollover a Postfix process might load
a private key and certificate from separate files that don't match.
Various operational errors may even result in a persistent broken con-
figuration in which the certificate does not match the private key.

The file or files must contain at most one key of each type. If, for
example, two or more RSA keys and corresponding chains are listed, de-
pending on the version of OpenSSL either only the last one will be used
or a configuration error may be detected. Note that while "Ed25519" and
"Ed448" are considered separate algorithms, the various ECDSA curves
(typically one of prime256v1, secp384r1 or secp521r1) are considered as
different parameters of a single "ECDSA" algorithm, so it is not
presently possible to configure keys for more than one ECDSA curve.

Example (separate files for each key and corresponding certificate
chain):

/etc/postfix/main.cf:
smtp_tls_chain_files =
${config_directory}/ed25519.pem,
${config_directory}/ed448.pem,
${config_directory}/rsa.pem

/etc/postfix/ed25519.pem:
-----BEGIN PRIVATE KEY-----
MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
...
nC0egv51YPDWxEHom4QA
-----END CERTIFICATE-----

/etc/postfix/ed448.pem:
-----BEGIN PRIVATE KEY-----
MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
...
pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
-----END CERTIFICATE-----

/etc/postfix/rsa.pem:
-----BEGIN PRIVATE KEY-----
MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
...
ahQkZ3+krcaJvDSMgvu0tDc=
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
...
Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
-----END CERTIFICATE-----

Example (all keys and certificates in a single file):

/etc/postfix/main.cf:
smtp_tls_chain_files = ${config_directory}/chains.pem

/etc/postfix/chains.pem:
-----BEGIN PRIVATE KEY-----
MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
...
nC0egv51YPDWxEHom4QA
-----END CERTIFICATE-----
-----BEGIN PRIVATE KEY-----
MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
...
pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
-----END CERTIFICATE-----
-----BEGIN PRIVATE KEY-----
MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
...
ahQkZ3+krcaJvDSMgvu0tDc=
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
...
Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
-----END CERTIFICATE-----

This feature is available in Postfix 3.4 and later.

smtp_tls_cipherlist (default: empty)
Obsolete Postfix < 2.3 control for the Postfix SMTP client TLS cipher
list. As this feature applies to all TLS security levels, it is easy to
create interoperability problems by choosing a non-default cipher list.
Do not use a non-default TLS cipher list on hosts that deliver email to
the public Internet: you will be unable to send email to servers that
only support the ciphers you exclude. Using a restricted cipher list may
be more appropriate for an internal MTA, where one can exert some con-
trol over the TLS software and settings of the peer servers.

Note: do not use "" quotes around the parameter value.

This feature is available in Postfix version 2.2. It is not used with
Postfix 2.3 and later; use smtp_tls_mandatory_ciphers instead.

smtp_tls_ciphers (default: medium)
The minimum TLS cipher grade that the Postfix SMTP client will use with
opportunistic TLS encryption. Cipher types listed in smtp_tls_ex-
clude_ciphers are excluded from the base definition of the selected ci-
pher grade. The default value is "medium" for Postfix releases after
the middle of 2015, "export" for older releases.

When TLS is mandatory the cipher grade is chosen via the smtp_tls_manda-
tory_ciphers configuration parameter, see there for syntax details. See
smtp_tls_policy_maps for information on how to configure ciphers on a
per-destination basis.

This feature is available in Postfix 2.6 and later. With earlier Postfix
releases only the smtp_tls_mandatory_ciphers parameter is implemented,
and opportunistic TLS always uses "export" or better (i.e. all) ciphers.

smtp_tls_connection_reuse (default: no)
Try to make multiple deliveries per TLS-encrypted connection. This uses
the tlsproxy(8) service to encrypt an SMTP connection, uses the
scache(8) service to save that connection, and relies on hints from the
qmgr(8) daemon.

See "Client-side TLS connection reuse" for background details.

This feature is available in Postfix 3.4 and later.

smtp_tls_dane_insecure_mx_policy (default: dane)
The TLS policy for MX hosts with "secure" TLSA records when the nexthop
destination security level is dane, but the MX record was found via an
"insecure" MX lookup. The choices are:

may The TLSA records will be ignored and TLS will be optional. If
the MX host does not appear to support STARTTLS, or the STARTTLS
handshake fails, mail may be sent in the clear.

encrypt
The TLSA records will signal a requirement to use TLS. While TLS
encryption will be required, authentication will not be per-
formed.

dane The TLSA records will be used just as with "secure" MX records.
TLS encryption will be required, and, if at least one of the TLSA
records is "usable", authentication will be required. When au-
thentication succeeds, it will be logged only as "Trusted", not
"Verified", because the MX host name could have been forged.
The default setting is "dane" as of Postfix versions 3.6.17,
3.7.13, 3.8.8, 3.9.2, and 3.10.0. With earlier versions the de-
fault was mistakenly dependent on the smtp_tls_security_level
setting.

Though with "insecure" MX records an active attacker can compromise SMTP
transport security by returning forged MX records, such attacks are
"tamper-evident" since any forged MX hostnames will be recorded in the
mail logs. Attackers who place a high value on staying hidden may be
deterred from forging MX records.

This feature is available in Postfix 3.1 and later. The may policy is
backwards-compatible with earlier Postfix versions.

smtp_tls_dcert_file (default: empty)
File with the Postfix SMTP client DSA certificate in PEM format. This
file may also contain the Postfix SMTP client private DSA key. The DSA
algorithm is obsolete and should not be used.

See the discussion under smtp_tls_cert_file for more details.

Example:

smtp_tls_dcert_file = /etc/postfix/client-dsa.pem

This feature is available in Postfix 2.2 and later.

smtp_tls_dkey_file (default: $smtp_tls_dcert_file)
File with the Postfix SMTP client DSA private key in PEM format. This
file may be combined with the Postfix SMTP client DSA certificate file
specified with $smtp_tls_dcert_file. The DSA algorithm is obsolete and
should not be used.

The private key must be accessible without a pass-phrase, i.e. it must
not be encrypted. File permissions should grant read-only access to the
system superuser account ("root"), and no access to anyone else.

This feature is available in Postfix 2.2 and later.

smtp_tls_eccert_file (default: empty)
File with the Postfix SMTP client ECDSA certificate in PEM format. This
file may also contain the Postfix SMTP client ECDSA private key. With
Postfix >= 3.4 the preferred way to configure client keys and certifi-
cates is via the "smtp_tls_chain_files" parameter.

See the discussion under smtp_tls_cert_file for more details.

Example:

smtp_tls_eccert_file = /etc/postfix/ecdsa-ccert.pem

This feature is available in Postfix 2.6 and later, when Postfix is com-
piled and linked with OpenSSL 1.0.0 or later.

smtp_tls_eckey_file (default: $smtp_tls_eccert_file)
File with the Postfix SMTP client ECDSA private key in PEM format. This
file may be combined with the Postfix SMTP client ECDSA certificate file
specified with $smtp_tls_eccert_file. With Postfix >= 3.4 the preferred
way to configure client keys and certificates is via the
"smtp_tls_chain_files" parameter.

This feature is available in Postfix 2.6 and later, when Postfix is com-
piled and linked with OpenSSL 1.0.0 or later.

smtp_tls_enable_rpk (default: no)
Request that remote SMTP servers send an RFC7250 raw public key instead
of an X.509 certificate. This feature and the enable_rpk policy at-
tribute are ignored when there is no raw public key support in the local
TLS implementation.

• At the "may", "encrypt" and "fingerprint" security levels, with
parameter setting "smtp_tls_enable_rpk = yes" or with "enable_rpk
= yes" in a policy entry, the Postfix SMTP client will indicate
in the TLS handshake that it prefers to receive a raw server pub-
lic key, but it will still accept a server public key certifi-
cate.

• At the "fingerprint" security level, with parameter setting
"smtp_tls_enable_rpk = yes" or with "enable_rpk = yes" in a pol-
icy entry, server authentication based on certificate finger-
prints becomes more fragile. Even if the server private key and
certificate remain unchanged, the remote SMTP server will fail
fingerprint authentication (won't match the configured list of
fingerprints) when it starts sending a raw public key instead of
a certificate, after its TLS implementation is updated with raw
public key support. Therefore, DO NOT enable raw public keys to
remote destinations authenticated by server certificate finger-
prints. You should enable raw public keys only for servers
matched via their public key fingerprint.

• At the "verify" and "secure" security levels, the Postfix SMTP
client always ignores the parameter setting smtp_tls_enable_rpk
or the enable_rpk policy attribute.

• At the opportunistic "dane" security level, the Postfix SMTP
client ignores the parameter setting smtp_tls_enable_rpk or the
enable_rpk policy attribute (but it will respect them when it
falls back to the "may" or "encrypt" level). When all valid TLSA
records specify only server public keys (no certificates) and the
local TLS implementation supports raw public keys, the client
will indicate in the TLS handshake that it prefers to receive a
raw public key, but it will still accept a public key certifi-
cate.

• At the mandatory "dane-only" security level, the Postfix SMTP
client always ignores the parameter setting smtp_tls_enable_rpk
or the enable_rpk policy attribute. When all valid TLSA records
specify only server public keys (no certificates) and the local
TLS implementation supports raw public keys, the client will in-
dicate in the TLS handshake that it prefers to receive a raw pub-
lic key, but it will still accept a public key certificate.

The Postfix SMTP client is always willing to send raw public keys to
servers that solicit them when a client certificate is configured and
the local TLS implementation supports raw public keys.

Sample commands to compute certificate and public key SHA256 digests:

# SHA256 digest of the first certificate in "cert.pem"
$ openssl x509 -in cert.pem -outform DER | openssl dgst -sha256 -c

# SHA256 digest of the SPKI of the first certificate in "cert.pem"
$ openssl x509 -in cert.pem -pubkey -noout |
openssl pkey -pubin -outform DER | openssl dgst -sha256 -c

# SHA256 digest of the SPKI of the first private key in "pkey.pem"
$ openssl pkey -in pkey.pem -pubout -outform DER |
openssl dgst -sha256 -c

This feature is available in Postfix 3.9 and later.

smtp_tls_enforce_peername (default: yes)
With mandatory TLS encryption, require that the remote SMTP server host-
name matches the information in the remote SMTP server certificate. As
of RFC 2487 the requirements for hostname checking for MTA clients are
not specified.

This option can be set to "no" to disable strict peer name checking.
This setting has no effect on sessions that are controlled via the
smtp_tls_per_site table.

Disabling the hostname verification can make sense in a closed environ-
ment where special CAs are created. If not used carefully, this option
opens the danger of a "man-in-the-middle" attack (the CommonName of this
attacker will be logged).

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and
later use smtp_tls_security_level instead.

smtp_tls_enforce_sts_mx_patterns (default: yes)
Transform the TLS policy from an STS policy plugin: connect to an MX
host only if its name matches any STS policy MX host pattern, and match
the server certificate against the MX hostname. This setting takes ef-
fect only when an STS policy plugin has TLSRPT support enabled, so that
it forwards STS policy attributes to Postfix. This works even if Postfix
TLSRPT support is disabled at build time or at runtime.

Without the above configuration settings for Postfix and STS plugins,
the old behavior stays in effect: connect to any MX host listed in DNS,
and match a server certificate against any STS policy MX host pattern.

This feature is available in Postfix >= 3.10.5.

smtp_tls_exclude_ciphers (default: empty)
List of ciphers or cipher types to exclude from the Postfix SMTP client
cipher list at all TLS security levels. This is not an OpenSSL ci-
pherlist, it is a simple list separated by whitespace and/or commas. The
elements are a single cipher, or one or more "+" separated cipher prop-
erties, in which case only ciphers matching all the properties are ex-
cluded.

Examples (some of these will cause problems):

smtp_tls_exclude_ciphers = aNULL
smtp_tls_exclude_ciphers = MD5, DES
smtp_tls_exclude_ciphers = DES+MD5
smtp_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
smtp_tls_exclude_ciphers = kEDH+aRSA

The first setting disables anonymous ciphers. The next setting disables
ciphers that use the MD5 digest algorithm or the (single) DES encryption
algorithm. The next setting disables ciphers that use MD5 and DES to-
gether. The next setting disables the two ciphers "AES256-SHA" and
"DES-CBC3-MD5". The last setting disables ciphers that use "EDH" key ex-
change with RSA authentication.

This feature is available in Postfix 2.3 and later.

smtp_tls_fingerprint_cert_match (default: empty)
List of acceptable remote SMTP server certificate fingerprints for the
"fingerprint" TLS security level (smtp_tls_security_level = finger-
print). At this security level, Certification Authorities are not used,
and certificate expiration times are ignored. Instead, server certifi-
cates are verified directly via their certificate fingerprint or public
key fingerprint (Postfix 2.9 and later). The fingerprint is a message
digest of the server certificate (or public key). The digest algorithm
is selected via the smtp_tls_fingerprint_digest parameter.

The colons between each pair of nibbles in the fingerprint value are op-
tional (Postfix >= 3.6). These were required in earlier Postfix re-
leases.

When an smtp_tls_policy_maps table entry specifies the "fingerprint" se-
curity level, any "match" attributes in that entry specify the list of
valid fingerprints for the corresponding destination. Multiple finger-
prints can be combined with a "|" delimiter in a single match attribute,
or multiple match attributes can be employed.

Example: Certificate fingerprint verification with internal mailhub.
Two matching fingerprints are listed. The relayhost may be multiple
physical hosts behind a load-balancer, each with its own private/public
key and self-signed certificate. Alternatively, a single relayhost may
be in the process of switching from one set of private/public keys to
another, and both keys are trusted just prior to the transition.

relayhost = [mailhub.example.com]
smtp_tls_security_level = fingerprint
smtp_tls_fingerprint_digest = sha256
smtp_tls_fingerprint_cert_match =
cd:fc:d8:db:f8:c4:82:96:6c:...:28:71:e8:f5:8d:a5:0d:9b:d4:a6
dd:5c:ef:f5:c3:bc:64:25:36:...:99:36:06:ce:40:ef:de:2e:ad:a4

Example: Certificate fingerprint verification with selected destina-
tions. As in the example above, we show two matching fingerprints:

/etc/postfix/main.cf:
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
smtp_tls_fingerprint_digest = sha256

/etc/postfix/tls_policy:
example.com fingerprint
match=51:e9:af:2e:1e:40:1f:...:64:0a:30:35:2d:09:16:31:5a:eb:82:76
match=b6:b4:72:34:e2:59:cd:...:c2:ca:63:0d:4d:cc:2c:7d:84:de:e6:2f

This feature is available in Postfix 2.5 and later.

smtp_tls_fingerprint_digest (default: see postconf -d output)
The message digest algorithm used to construct remote SMTP server cer-
tificate fingerprints. At the "fingerprint" TLS security level
(smtp_tls_security_level = fingerprint), the server certificate is veri-
fied by directly matching its certificate fingerprint or its public key
fingerprint (Postfix 2.9 and later). The fingerprint is the message di-
gest of the server certificate (or its public key) using the selected
algorithm. With a digest algorithm resistant to "second pre-image" at-
tacks, it is not feasible to create a new public key and a matching cer-
tificate (or public/private key-pair) that has the same fingerprint.

The default algorithm is sha256 with Postfix >= 3.6 and the compatibil-
ity_level set to 3.6 or higher. With Postfix <= 3.5, the default algo-
rithm is md5.

The best-practice algorithm is now sha256. Recent advances in hash func-
tion cryptanalysis have led to md5 and sha1 being deprecated in favor of
sha256. However, as long as there are no known "second pre-image" at-
tacks against the older algorithms, their use in this context, though
not recommended, is still likely safe.

While additional digest algorithms are often available with OpenSSL's
libcrypto, only those used by libssl in SSL cipher suites are available
to Postfix. You'll likely find support for md5, sha1, sha256 and
sha512.

To find the fingerprint of a specific certificate file, with a specific
digest algorithm, run:

$ openssl x509 -noout -fingerprint -digest -in certfile.pem

The text to the right of the "=" sign is the desired fingerprint. For
example:

$ openssl x509 -noout -fingerprint -sha256 -in cert.pem
SHA256 Fingerprint=D4:6A:AB:19:24:...:BB:A6:CB:66:82:C0:8E:9B:EE:29:A8:1A

To extract the public key fingerprint from an X.509 certificate, you
need to extract the public key from the certificate and compute the ap-
propriate digest of its DER (ASN.1) encoding. With OpenSSL the "-pubkey"
option of the "x509" command extracts the public key always in "PEM"
format. We pipe the result to another OpenSSL command that converts the
key to DER and then to the "dgst" command to compute the fingerprint.

The actual command to transform the key to DER format depends on the
version of OpenSSL used. As of OpenSSL 1.0.0, the "pkey" command sup-
ports all key types.

# OpenSSL >= 1.0 with SHA-256 fingerprints.
$ openssl x509 -in cert.pem -noout -pubkey |
openssl pkey -pubin -outform DER |
openssl dgst -sha256 -c
(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:...:fc:09:1a:61:98:b5:bc:7c:60:58

The Postfix SMTP server and client log the peer (leaf) certificate fin-
gerprint and the public key fingerprint when the TLS loglevel is 2 or
higher.

This feature is available in Postfix 2.5 and later.

smtp_tls_force_insecure_host_tlsa_lookup (default: no)
Lookup the associated DANE TLSA RRset even when a hostname is not an
alias and its address records lie in an unsigned zone. This is unlikely
to ever yield DNSSEC validated results, since child zones of unsigned
zones are also unsigned in the absence of DLV or locally configured
non-root trust-anchors. We anticipate that such mechanisms will not be
used for just the "_tcp" subdomain of a host. Suppressing the TLSA
RRset lookup reduces latency and avoids potential interoperability prob-
lems with nameservers for unsigned zones that are not prepared to handle
the new TLSA RRset.

This feature is available in Postfix 2.11.

smtp_tls_key_file (default: $smtp_tls_cert_file)
File with the Postfix SMTP client RSA private key in PEM format. This
file may be combined with the Postfix SMTP client RSA certificate file
specified with $smtp_tls_cert_file. With Postfix >= 3.4 the preferred
way to configure client keys and certificates is via the
"smtp_tls_chain_files" parameter.

Example:

smtp_tls_key_file = $smtp_tls_cert_file

This feature is available in Postfix 2.2 and later.

smtp_tls_loglevel (default: 0)
Enable additional Postfix SMTP client logging of TLS activity. Each
logging level also includes the information that is logged at a lower
logging level.

0 Disable logging of TLS activity.

1 Log only a summary message on TLS handshake completion - no
logging of remote SMTP server certificate trust-chain verifica-
tion errors if server certificate verification is not required.
With Postfix 2.8 and earlier, log the summary message and uncon-
ditionally log trust-chain verification errors.

2 Also enable verbose logging in the Postfix TLS library, log
session cache operations, and enable OpenSSL logging of the
progress of the SSL handshake.

3 Also log the hexadecimal and ASCII dump of the TLS negotiation
process.

4 Also log the hexadecimal and ASCII dump of complete transmis-
sion after STARTTLS.

Do not use "smtp_tls_loglevel = 2" or higher except in case of problems.
Use of loglevel 4 is strongly discouraged.

This feature is available in Postfix 2.2 and later.

smtp_tls_mandatory_ciphers (default: medium)
The minimum TLS cipher grade that the Postfix SMTP client will use with
mandatory TLS encryption. The default value "medium" is suitable for
most destinations with which you may want to enforce TLS, and is beyond
the reach of today's cryptanalytic methods. See smtp_tls_policy_maps for
information on how to configure ciphers on a per-destination basis.

The following cipher grades are supported:

high Enable only "HIGH" grade OpenSSL ciphers. This setting may be
appropriate when all mandatory TLS destinations (e.g. when all
mail is routed to a suitably capable relayhost) support at least
one "HIGH" grade cipher. The underlying cipherlist is specified
via the tls_high_cipherlist configuration parameter, which you
are strongly encouraged not to change.

medium Enable "MEDIUM" grade or better OpenSSL ciphers. The underlying
cipherlist is specified via the tls_medium_cipherlist configura-
tion parameter, which you are strongly encouraged not to change.

null Enable only the "NULL" OpenSSL ciphers, these provide authentica-
tion without encryption. This setting is only appropriate in the
rare case that all servers are prepared to use NULL ciphers (not
normally enabled in TLS servers). A plausible use-case is an LMTP
server listening on a UNIX-domain socket that is configured to
support "NULL" ciphers. The underlying cipherlist is specified
via the tls_null_cipherlist configuration parameter, which you
are strongly encouraged not to change.

low Enable "LOW" grade or stronger OpenSSL ciphers. In Postfix >=
3.8 this cipher grade is always identical to "medium". Recent
versions of OpenSSL do not support any "LOW" grade ciphers. In
earlier Postfix releases the underlying cipherlist was specified
via the tls_low_cipherlist configuration parameter, which you are
strongly encouraged not to change. This obsolete cipher grade
SHOULD NOT be used.

export Enable "EXPORT" grade or stronger OpenSSL ciphers. In Postfix >=
3.8 this cipher grade is always identical to "medium". Recent
versions of OpenSSL do not support any "EXPORT" grade ciphers.
In earlier Postfix releases the underlying cipherlist was speci-
fied via the tls_export_cipherlist configuration parameter, which
you are strongly encouraged not to change. This obsolete cipher
grade SHOULD NOT be used.

The underlying cipherlists for grades other than "null" include anony-
mous ciphers, but these are automatically filtered out if the Postfix
SMTP client is configured to verify server certificates. You are very
unlikely to need to take any steps to exclude anonymous ciphers, they
are excluded automatically as necessary. If you must exclude anonymous
ciphers at the "may" or "encrypt" security levels, when the Postfix SMTP
client does not need or use peer certificates, set "smtp_tls_exclude_ci-
phers = aNULL". To exclude anonymous ciphers only when TLS is enforced,
set "smtp_tls_mandatory_exclude_ciphers = aNULL".

This feature is available in Postfix 2.3 and later.

smtp_tls_mandatory_exclude_ciphers (default: empty)
Additional list of ciphers or cipher types to exclude from the Postfix
SMTP client cipher list at mandatory TLS security levels. This list
works in addition to the exclusions listed with smtp_tls_exclude_ciphers
(see there for syntax details).

Starting with Postfix 2.6, the mandatory cipher exclusions can be speci-
fied on a per-destination basis via the TLS policy "exclude" attribute.
See smtp_tls_policy_maps for notes and examples.

This feature is available in Postfix 2.3 and later.

smtp_tls_mandatory_protocols (default: see postconf -d output)
TLS protocols that the Postfix SMTP client will use with mandatory TLS
encryption. In main.cf the values are separated by whitespace, commas
or colons. In the policy table "protocols" attribute (see smtp_tls_pol-
icy_maps) the only valid separator is colon. An empty value means allow
all protocols.

The valid protocol names (see SSL_get_version(3)) are "SSLv2", "SSLv3",
"TLSv1", "TLSv1.1", "TLSv1.2" and "TLSv1.3". Starting with Postfix 3.6,
the default value is ">=TLSv1", which sets TLS 1.0 as the lowest sup-
ported TLS protocol version (see below). Older releases use the "!" ex-
clusion syntax, also described below.

As of Postfix 3.6, the preferred way to limit the range of acceptable
protocols is to set a lowest acceptable TLS protocol version and/or a
highest acceptable TLS protocol version. To set the lower bound include
an element of the form: ">=version" where version is a either one of the
TLS protocol names listed above, or a hexadecimal number corresponding
to the desired TLS protocol version (0301 for TLS 1.0, 0302 for TLS 1.1,
etc.). For the upper bound, use "<=version". There must be no white-
space between the ">=" or "<=" symbols and the protocol name or number.

Hexadecimal protocol numbers make it possible to specify protocol bounds
for TLS versions that are known to OpenSSL, but might not be known to
Postfix. They cannot be used with the legacy exclusion syntax. Leading
"0" or "0x" prefixes are supported, but not required. Therefore, "301",
"0301", "0x301" and "0x0301" are all equivalent to "TLSv1". Hexadecimal
versions unknown to OpenSSL will fail to set the upper or lower bound,
and a warning will be logged. Hexadecimal versions should only be used
when Postfix is linked with some future version of OpenSSL that supports
TLS 1.4 or later, but Postfix does not yet support a symbolic name for
that protocol version.

Hexadecimal example (Postfix >= 3.6):

# Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtp_tls_mandatory_protocols = >=TLSv1.2, <=0305
# Allow only TLS 1.2 and up:
smtp_tls_mandatory_protocols = >=0x0303

With Postfix < 3.6 there is no support for a minimum or maximum version,
and the protocol range is configured via protocol exclusions. To re-
quire at least TLS 1.0, set "smtp_tls_mandatory_protocols = !SSLv2,
!SSLv3". Listing the protocols to include, rather than the protocols to
exclude, is supported, but not recommended. The exclusion syntax more
accurately matches the underlying OpenSSL interface.

When using the exclusion syntax, take care to ensure that the range of
protocols supported by the Postfix SMTP client is contiguous. When a
protocol version is enabled, disabling any higher version implicitly
disables all versions above that higher version. Thus, for example:

smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1.1

also disables any protocol versions higher than TLSv1.1 leaving only
"TLSv1" enabled.

Support for "TLSv1.3" was introduced in OpenSSL 1.1.1. Disabling this
protocol via "!TLSv1.3" is supported since Postfix 3.4 (or patch re-
leases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).

While the vast majority of SMTP servers with DANE TLSA records now sup-
port at least TLS 1.2, a few still only support TLS 1.0. If you use
"dane" or "dane-only" it is best not to disable TLSv1, except perhaps
via the policy table for destinations which you are sure will support
"TLSv1.2".

See the documentation of the smtp_tls_policy_maps parameter and
TLS_README for more information about security levels.

Example:
# Preferred syntax with Postfix >= 3.6:
smtp_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
# Legacy syntax:
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

This feature is available in Postfix 2.3 and later.

smtp_tls_note_starttls_offer (default: no)
Log the hostname of a remote SMTP server that offers STARTTLS, when TLS
is not already enabled for that server.

The logfile record looks like:

postfix/smtp[pid]: Host offered STARTTLS: [name.of.host]

This feature is available in Postfix 2.2 and later.

smtp_tls_per_site (default: empty)
Optional lookup tables with the Postfix SMTP client TLS usage policy by
next-hop destination and by remote SMTP server hostname. When both
lookups succeed, the more specific per-site policy (NONE, MUST, etc.)
overrides the less specific one (MAY), and the more secure per-site pol-
icy (MUST, etc.) overrides the less secure one (NONE). With Postfix 2.3
and later smtp_tls_per_site is strongly discouraged: use smtp_tls_pol-
icy_maps instead.

Use of the bare hostname as the per-site table lookup key is discour-
aged. Always use the full destination nexthop (enclosed in [] with a
possible ":port" suffix). A recipient domain or MX-enabled transport
next-hop with no port suffix may look like a bare hostname, but is still
a suitable destination.

Specify a next-hop destination or server hostname on the left-hand side;
no wildcards are allowed. The next-hop destination is either the recipi-
ent domain, or the destination specified with a transport(5) table, the
relayhost parameter, or the relay_transport parameter. On the right
hand side specify one of the following keywords:

NONE Don't use TLS at all. This overrides a less specific MAY lookup
result from the alternate host or next-hop lookup key, and over-
rides the global smtp_use_tls, smtp_enforce_tls, and smtp_tls_en-
force_peername settings.

MAY Try to use TLS if the server announces support, otherwise use an
unencrypted connection; after a failed TLS handshake or TLS ses-
sion, fall back to plaintext if the message has spent mini-
mal_backoff_time in the mail queue. This level has less prece-
dence than a more specific result (including NONE) from the al-
ternate host or next-hop lookup key, and has less precedence than
the more specific global "smtp_enforce_tls = yes" or
"smtp_tls_enforce_peername = yes".

MUST_NOPEERMATCH
Require TLS encryption, but do not require that the remote SMTP
server hostname matches the information in the remote SMTP server
certificate, or that the server certificate was issued by a
trusted CA. This overrides a less secure NONE or a less specific
MAY lookup result from the alternate host or next-hop lookup key,
and overrides the global smtp_use_tls, smtp_enforce_tls and
smtp_tls_enforce_peername settings.

MUST Require TLS encryption, require that the remote SMTP server host-
name matches the information in the remote SMTP server certifi-
cate, and require that the remote SMTP server certificate was is-
sued by a trusted CA. This overrides a less secure NONE or
MUST_NOPEERMATCH or a less specific MAY lookup result from the
alternate host or next-hop lookup key, and overrides the global
smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername set-
tings.

The above keywords correspond to the "none", "may", "encrypt" and "ver-
ify" security levels for the new smtp_tls_security_level parameter in-
troduced in Postfix 2.3. Starting with Postfix 2.3, and independently of
how the policy is specified, the smtp_tls_mandatory_ciphers and
smtp_tls_mandatory_protocols parameters apply when TLS encryption is
mandatory. Connections for which encryption is optional typically enable
all "export" grade and better ciphers (see smtp_tls_ciphers and
smtp_tls_protocols).

As long as no secure DNS lookup mechanism is available, false hostnames
in MX or CNAME responses can change the server hostname that Postfix
uses for TLS policy lookup and server certificate verification. Even
with a perfect match between the server hostname and the server certifi-
cate, there is no guarantee that Postfix is connected to the right
server. See TLS_README (Closing a DNS loophole with obsolete per-site
TLS policies) for a possible work-around.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and
later use smtp_tls_policy_maps instead.

smtp_tls_policy_maps (default: empty)
Optional lookup tables with the Postfix SMTP client TLS security policy
by next-hop destination; when a non-empty value is specified, this over-
rides the obsolete smtp_tls_per_site parameter. See TLS_README for a
more detailed discussion of TLS security levels, and see TLSRPT_README
for additional configuration that may be needed for MTA-STS plugins.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

The TLS policy table is indexed by the full next-hop destination, which
is either the recipient domain, or the verbatim next-hop specified in
the transport table, $local_transport, $virtual_transport, $relay_trans-
port or $default_transport. This includes any enclosing square brackets
and any non-default destination server port suffix. The LMTP socket type
prefix (inet: or unix:) is not included in the lookup key.

Only the next-hop domain, or $myhostname with LMTP over UNIX-domain
sockets, is used as the nexthop name for certificate verification. The
port and any enclosing square brackets are used in the table lookup key,
but are not used for server name verification.

When the lookup key is a domain name without enclosing square brackets
or any :port suffix (typically the recipient domain), and the full do-
main is not found in the table, just as with the transport(5) table, the
parent domain starting with a leading "." is matched recursively. This
allows one to specify a security policy for a recipient domain and all
its sub-domains.

The lookup result is a security level, followed by an optional list of
whitespace and/or comma separated name=value attributes that override
related main.cf settings. The TLS security levels in order of increasing
security are:

none No TLS. No additional attributes are supported at this level.

may Opportunistic TLS. Since sending in the clear is acceptable, de-
manding stronger than default TLS security merely reduces inter-
operability. The optional "ciphers", "exclude", and "protocols"
attributes (available for opportunistic TLS with Postfix >= 2.6)
and "connection_reuse" attribute (Postfix >= 3.4) override the
"smtp_tls_ciphers", "smtp_tls_exclude_ciphers", "smtp_tls_proto-
cols", and "smtp_tls_connection_reuse" configuration parameters.
In the policy table, multiple ciphers, protocols or excluded ci-
phers must be separated by colons, as attribute values may not
contain whitespace or commas. At this level and higher, the op-
tional "servername" attribute (available with Postfix >= 3.4)
overrides the global "smtp_tls_servername" parameter, enabling
per-destination configuration of the SNI extension sent to the
remote SMTP server. The optional "enable_rpk" attribute (Postfix
>= 3.9) overrides the main.cf smtp_tls_enable_rpk parameter.
When opportunistic TLS handshakes fail, Postfix retries the con-
nection with TLS disabled. This allows mail delivery to sites
with non-interoperable TLS implementations.

encrypt
Mandatory TLS encryption. Mail is delivered only if the remote
SMTP server offers STARTTLS and the TLS handshake succeeds. At
this level and higher, the optional "protocols" attribute over-
rides the main.cf smtp_tls_mandatory_protocols parameter, the op-
tional "ciphers" attribute overrides the main.cf smtp_tls_manda-
tory_ciphers parameter, the optional "exclude" attribute (Postfix
>= 2.6) overrides the main.cf smtp_tls_mandatory_exclude_ciphers
parameter, and the optional "connection_reuse" attribute (Postfix
>= 3.4) overrides the main.cf smtp_tls_connection_reuse parame-
ter. In the policy table, multiple ciphers, protocols or excluded
ciphers must be separated by colons, as attribute values may not
contain whitespace or commas. The optional "enable_rpk" at-
tribute (Postfix >= 3.9) overrides the main.cf smtp_tls_en-
able_rpk parameter.

dane Opportunistic DANE TLS. The TLS policy for the destination is
obtained via TLSA records in DNSSEC. If no TLSA records are
found, the effective security level used is may. If TLSA records
are found, but none are usable, the effective security level is
encrypt. When usable TLSA records are obtained for the remote
SMTP server, the server certificate must match the TLSA records.
RFC 7672 (DANE) TLS authentication and DNSSEC support is avail-
able with Postfix 2.11 and later. The optional "connection_reuse"
attribute (Postfix >= 3.4) overrides the main.cf smtp_tls_connec-
tion_reuse parameter. When the effective security level used is
may, the optional "ciphers", "exclude", and "protocols" attrib-
utes (Postfix >= 2.6) override the "smtp_tls_ciphers",
"smtp_tls_exclude_ciphers", and "smtp_tls_protocols" configura-
tion parameters. When the effective security level used is en-
crypt, the optional "ciphers", "exclude", and "protocols" attrib-
utes (Postfix >= 2.6) override the "smtp_tls_mandatory_ciphers",
"smtp_tls_mandatory_exclude_ciphers", and "smtp_tls_manda-
tory_protocols" configuration parameters.

dane-only
Mandatory DANE TLS. The TLS policy for the destination is ob-
tained via TLSA records in DNSSEC. If no TLSA records are found,
or none are usable, no connection is made to the server. When
usable TLSA records are obtained for the remote SMTP server, the
server certificate must match the TLSA records. RFC 7672 (DANE)
TLS authentication and DNSSEC support is available with Postfix
2.11 and later. The optional "ciphers", "exclude", and "proto-
cols" attributes (Postfix >= 2.6) override the "smtp_tls_manda-
tory_ciphers", "smtp_tls_mandatory_exclude_ciphers", and
"smtp_tls_mandatory_protocols" configuration parameters. The op-
tional "connection_reuse" attribute (Postfix >= 3.4) overrides
the main.cf smtp_tls_connection_reuse parameter.

fingerprint
Certificate fingerprint verification. Available with Postfix 2.5
and later. At this security level, there are no trusted Certifi-
cation Authorities. The certificate trust chain, expiration date,
... are not checked. Instead, the optional policy table "match"
attribute, or else the main.cf smtp_tls_fingerprint_cert_match
parameter, lists the certificate fingerprints or the public key
fingerprints (Postfix 2.9 and later) of acceptable server cer-
tificates. The digest algorithm used to calculate the fingerprint
is selected by the smtp_tls_fingerprint_digest parameter. Multi-
ple fingerprints can be combined with a "|" delimiter in a single
match attribute, or multiple match attributes can be employed.
The ":" character is not used as a delimiter as it occurs between
each pair of fingerprint (hexadecimal) digits. The optional "ci-
phers", "exclude", and "protocols" attributes (Postfix >= 2.6)
override the "smtp_tls_mandatory_ciphers", "smtp_tls_manda-
tory_exclude_ciphers", and "smtp_tls_mandatory_protocols" config-
uration parameters. The optional "connection_reuse" attribute
(Postfix >= 3.4) overrides the main.cf smtp_tls_connection_reuse
parameter. The optional "enable_rpk" attribute (Postfix >= 3.9)
overrides the main.cf smtp_tls_enable_rpk parameter.

verify Mandatory TLS verification. Mail is delivered only if the TLS
handshake succeeds, the remote SMTP server certificate chain can
be validated, and a DNS name in the certificate matches the spec-
ified match criteria. At this security level, DNS MX lookups are
presumed to be secure enough, and the name verified in the server
certificate is potentially obtained via unauthenticated DNS MX
lookups. The optional "match" attribute overrides the main.cf
smtp_tls_verify_cert_match parameter. In the policy table, multi-
ple match patterns and strategies must be separated by colons.
In practice explicit control over matching is more common with
the "secure" policy, described below. The optional "ciphers",
"exclude", and "protocols" attributes (Postfix >= 2.6) override
the "smtp_tls_mandatory_ciphers", "smtp_tls_mandatory_exclude_ci-
phers", and "smtp_tls_mandatory_protocols" configuration parame-
ters. With Postfix >= 2.11 the optional "tafile" policy table at-
tribute modifies trust chain verification in the same manner as
the "smtp_tls_trust_anchor_file" parameter. The "tafile" at-
tribute may be specified multiple times to load multiple
trust-anchor files. The optional "connection_reuse" attribute
(Postfix >= 3.4) overrides the main.cf smtp_tls_connection_reuse
parameter.

secure Secure certificate verification. Mail is delivered only if the
TLS handshake succeeds, the remote SMTP server certificate chain
can be validated, and a DNS name in the certificate matches the
specified match criteria. At this security level, DNS MX
lookups, though potentially used to determine the candidate
next-hop gateway IP addresses, are not presumed to be secure
enough for TLS peername verification. Instead, the default name
verified in the server certificate is obtained directly from the
next-hop, or is explicitly specified via the optional "match" at-
tribute which overrides the main.cf smtp_tls_secure_cert_match
parameter. In the policy table, multiple match patterns and
strategies must be separated by colons. The match attribute is
most useful when multiple domains are supported by a common
server: the policy entries for additional domains specify match-
ing rules for the primary domain certificate. While transport ta-
ble overrides that route the secondary domains to the primary
nexthop also allow secure verification, they risk delivery to the
wrong destination when domains change hands or are re-assigned to
new gateways. With the "match" attribute approach, routing is not
perturbed, and mail is deferred if verification of a new MX host
fails. The optional "ciphers", "exclude", and "protocols" attrib-
utes (Postfix >= 2.6) override the "smtp_tls_mandatory_ciphers",
"smtp_tls_mandatory_exclude_ciphers", and "smtp_tls_manda-
tory_protocols" configuration parameters. With Postfix >= 2.11
the "tafile" attribute optionally modifies trust chain verifica-
tion in the same manner as the "smtp_tls_trust_anchor_file" para-
meter. The "tafile" attribute may be specified multiple times to
load multiple trust-anchor files. The optional "connection_reuse"
attribute (Postfix >= 3.4) overrides the main.cf smtp_tls_connec-
tion_reuse parameter.

Example:

/etc/postfix/main.cf:
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
# Postfix 2.5 and later.
#
# The default digest is sha256 with Postfix >= 3.6 and
# compatibility level >= 3.
#
smtp_tls_fingerprint_digest = sha256

/etc/postfix/tls_policy:
example.edu none
example.mil may
example.gov encrypt protocols=TLSv1
example.com verify ciphers=high
example.net secure
.example.net secure match=.example.net:example.net
[mail.example.org]:587 secure match=nexthop
# Postfix 2.5 and later
[thumb.example.org] fingerprint
match=b6:b4:72:34:e2:59:cd:...:c2:ca:63:0d:4d:cc:2c:7d:84:de:e6:2f
match=51:e9:af:2e:1e:40:1f:...:64:0a:30:35:2d:09:16:31:5a:eb:82:76

Note: The "hostname" strategy if listed in a non-default setting of
smtp_tls_secure_cert_match or in the "match" attribute in the policy ta-
ble can render the "secure" level vulnerable to DNS forgery. Do not use
the "hostname" strategy for secure-channel configurations in environ-
ments where DNS security is not assured.

This feature is available in Postfix 2.3 and later.

smtp_tls_protocols (default: see 'postconf -d' output)
TLS protocols that the Postfix SMTP client will use with opportunistic
TLS encryption. In main.cf the values are separated by whitespace, com-
mas or colons. In the policy table "protocols" attribute (see
smtp_tls_policy_maps) the only valid separator is colon. An empty value
means allow all protocols.

As of Postfix 3.6, the preferred way to limit the range of acceptable
protocols is to set the lowest acceptable TLS protocol version and/or
the highest acceptable TLS protocol version. To set the lower bound in-
clude an element of the form: ">=version" where version is either one of
the TLS protocol names listed above, or a hexadecimal number correspond-
ing to the desired TLS protocol version (0301 for TLS 1.0, 0302 for TLS
1.1, etc.). For the upper bound, use "<=version". There must be no
whitespace between the ">=" or "<=" symbols and the protocol name or
number.

Hexadecimal example (Postfix >= 3.6):

# Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtp_tls_protocols = >=TLSv1, <=0305
# Allow only TLS 1.0 and up:
smtp_tls_protocols = >=0x0301

With Postfix < 3.6 there is no support for a minimum or maximum version,
and the protocol range is configured via protocol exclusions. To re-
quire at least TLS 1.0, set "smtp_tls_protocols = !SSLv2, !SSLv3".
Listing the protocols to include, rather than protocols to exclude, is
supported, but not recommended. The exclusion form more accurately
matches the underlying OpenSSL interface.

When using the exclusion syntax, take care to ensure that the range of
protocols advertised by an SSL/TLS client is contiguous. When a proto-
col version is enabled, disabling any higher version implicitly disables
all versions above that higher version. Thus, for example:

smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1.1
also disables any protocols version higher than TLSv1.1 leaving only
"TLSv1" enabled.

Support for "TLSv1.3" was introduced in OpenSSL 1.1.1. Disabling this
protocol via "!TLSv1.3" is supported since Postfix 3.4 (or patch re-
leases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).

Example:
# Preferred syntax with Postfix >= 3.6:
smtp_tls_protocols = >=TLSv1, <=TLSv1.3
# Legacy syntax:
smtp_tls_protocols = !SSLv2, !SSLv3

This feature is available in Postfix 2.6 and later.

smtp_tls_scert_verifydepth (default: 9)
The verification depth for remote SMTP server certificates. A depth of 1
is sufficient if the issuing CA is listed in a local CA file.

The default verification depth is 9 (the OpenSSL default) for compati-
bility with earlier Postfix behavior. Prior to Postfix 2.5, the default
value was 5, but the limit was not actually enforced. If you have set
this to a lower non-default value, certificates with longer trust chains
may now fail to verify. Certificate chains with 1 or 2 CAs are common,
deeper chains are more rare and any number between 5 and 9 should suf-
fice in practice. You can choose a lower number if, for example, you
trust certificates directly signed by an issuing CA but not any CAs it
delegates to.

This feature is available in Postfix 2.2 and later.

smtp_tls_secure_cert_match (default: nexthop, dot-nexthop)
How the Postfix SMTP client verifies the server certificate peername for
the "secure" TLS security level. In a "secure" TLS policy table
($smtp_tls_policy_maps) entry the optional "match" attribute overrides
this main.cf setting.

This parameter specifies one or more patterns or strategies separated by
commas, whitespace or colons. In the policy table the only valid sepa-
rator is the colon character.

For a description of the pattern and strategy syntax see the
smtp_tls_verify_cert_match parameter. The "hostname" strategy should be
avoided in this context, as in the absence of a secure global DNS, using
the results of MX lookups in certificate verification is not immune to
active (man-in-the-middle) attacks on DNS.

Sample main.cf setting:

smtp_tls_secure_cert_match = nexthop

Sample policy table override:

example.net secure match=example.com:.example.com
.example.net secure match=example.com:.example.com

This feature is available in Postfix 2.3 and later.

smtp_tls_security_level (default: empty)
The default SMTP TLS security level for the Postfix SMTP client. When a
non-empty value is specified, this overrides the obsolete parameters
smtp_use_tls, smtp_enforce_tls, and smtp_tls_enforce_peername; when no
value is specified for smtp_tls_enforce_peername or the obsolete parame-
ters, the default SMTP TLS security level is none.

Specify one of the following security levels:

none No TLS. TLS will not be used unless enabled for specific destina-
tions via smtp_tls_policy_maps.

may Opportunistic TLS. Use TLS if this is supported by the remote
SMTP server, otherwise use plaintext; after a failed TLS hand-
shake or TLS session, fall back to plaintext if the message has
spent minimal_backoff_time in the mail queue. Since sending in
the clear is acceptable, demanding stronger than default TLS se-
curity merely reduces interoperability. The "smtp_tls_ciphers"
and "smtp_tls_protocols" (Postfix >= 2.6) configuration parame-
ters provide control over the protocols and cipher grade used
with opportunistic TLS. With earlier releases the opportunistic
TLS cipher grade is always "export" and no protocols are dis-
abled. When TLS handshakes fail, the connection is retried with
TLS disabled. This allows mail delivery to sites with non-inter-
operable TLS implementations.

encrypt
Mandatory TLS encryption. Since a minimum level of security is
intended, it is reasonable to be specific about sufficiently se-
cure protocol versions and ciphers. At this security level and
higher, the main.cf parameters smtp_tls_mandatory_protocols and
smtp_tls_mandatory_ciphers specify the TLS protocols and minimum
cipher grade which the administrator considers secure enough for
mandatory encrypted sessions. This security level is not an ap-
propriate default for systems delivering mail to the Internet.

dane Opportunistic DANE TLS. At this security level, the TLS policy
for the destination is obtained via DNSSEC. For TLSA policy to
be in effect, the destination domain's containing DNS zone must
be signed and the Postfix SMTP client's operating system must be
configured to send its DNS queries to a recursive DNS nameserver
that is able to validate the signed records. Each MX host's DNS
zone should also be signed, and should publish DANE TLSA (RFC
7672) records that specify how that MX host's TLS certificate is
to be verified. TLSA records do not preempt the normal SMTP MX
host selection algorithm, if some MX hosts support TLSA and oth-
ers do not, TLS security will vary from delivery to delivery. It
is up to the domain owner to configure their MX hosts and their
DNS sensibly. To configure the Postfix SMTP client for DNSSEC
lookups see the documentation for the smtp_dns_support_level
main.cf parameter. When DNSSEC-validated TLSA records are not
found the effective tls security level is "may". When TLSA
records are found, but are all unusable the effective security
level is "encrypt". For purposes of protocol and cipher selec-
tion, the "dane" security level is treated like a "mandatory" TLS
security level, and weak ciphers and protocols are disabled.
Since DANE authenticates server certificates the "aNULL" ci-
pher-suites are transparently excluded at this level, no need to
configure this manually. RFC 7672 (DANE) TLS authentication is
available with Postfix 2.11 and later.

dane-only
Mandatory DANE TLS. This is just like "dane" above, but DANE
TLSA authentication is required. There is no fallback to "may"
or "encrypt" when TLSA records are missing or unusable. RFC 7672
(DANE) TLS authentication is available with Postfix 2.11 and
later.

fingerprint
Certificate fingerprint verification. At this security level,
there are no trusted Certification Authorities. The certificate
trust chain, expiration date, etc., are not checked. Instead, the
smtp_tls_fingerprint_cert_match parameter lists the certificate
fingerprint or public key fingerprint (Postfix 2.9 and later) of
the valid server certificate. The digest algorithm used to calcu-
late the fingerprint is selected by the smtp_tls_fingerprint_di-
gest parameter. Available with Postfix 2.5 and later.

verify Mandatory TLS verification. At this security level, DNS MX
lookups are trusted to be secure enough, and the name verified in
the server certificate is usually obtained indirectly via unau-
thenticated DNS MX lookups. The smtp_tls_verify_cert_match para-
meter controls how the server name is verified. In practice ex-
plicit control over matching is more common at the "secure"
level, described below. This security level is not an appropriate
default for systems delivering mail to the Internet.

secure Secure-channel TLS. At this security level, DNS MX lookups,
though potentially used to determine the candidate next-hop gate-
way IP addresses, are not trusted to be secure enough for TLS
peername verification. Instead, the default name verified in the
server certificate is obtained from the next-hop domain as speci-
fied in the smtp_tls_secure_cert_match configuration parameter.
The default matching rule is that a server certificate matches
when its name is equal to or is a sub-domain of the nexthop do-
main. This security level is not an appropriate default for sys-
tems delivering mail to the Internet.

Examples:

# No TLS. Formerly: smtp_use_tls=no and smtp_enforce_tls=no.
smtp_tls_security_level = none

# Opportunistic TLS.
smtp_tls_security_level = may
# Do not tweak opportunistic ciphers or protocols unless it is essential
# to do so (if a security vulnerability is found in the SSL library that
# can be mitigated by disabling a particular protocol or raising the
# cipher grade).
smtp_tls_ciphers = medium
smtp_tls_protocols = >=TLSv1
# Legacy (Postfix < 3.6) syntax:
smtp_tls_protocols = !SSLv2, !SSLv3

# Mandatory (high-grade) TLS encryption.
smtp_tls_security_level = encrypt
smtp_tls_mandatory_ciphers = high

# Authenticated TLS 1.2 or better matching the nexthop domain or a
# subdomain.
smtp_tls_security_level = secure
smtp_tls_mandatory_ciphers = high
smtp_tls_mandatory_protocols = >=TLSv1.2
smtp_tls_secure_cert_match = nexthop, dot-nexthop

# Certificate fingerprint verification (Postfix >= 2.5).
# The CA-less "fingerprint" security level only scales to a limited
# number of destinations. As a global default rather than a per-site
# setting, this is practical only when mail for all recipients is sent
# to a central mail hub.
relayhost = [mailhub.example.com]
smtp_tls_security_level = fingerprint
smtp_tls_mandatory_protocols = >=TLSv1.2
smtp_tls_mandatory_ciphers = high
smtp_tls_fingerprint_cert_match =
3D:95:34:51:...:40:99:C0:C1
EC:3B:2D:B0:...:A3:9D:72:F6

This feature is available in Postfix 2.3 and later.

smtp_tls_servername (default: empty)
Optional name to send to the remote SMTP server in the TLS Server Name
Indication (SNI) extension. The SNI extension is always on when DANE is
used to authenticate the server, and in that case the SNI name sent is
the one required by RFC7672 and this parameter is ignored.

Some SMTP servers use the received SNI name to select an appropriate
certificate chain to present to the client. While this may improve in-
teroperability with such servers, it may reduce interoperability with
other servers that choose to abort the connection when they don't have a
certificate chain configured for the requested name. Such servers
should select a default certificate chain and continue the handshake,
but some may not. Therefore, absent DANE, no SNI name is sent by de-
fault.

The SNI name must be either a valid DNS hostname, or else one of the
special values hostname or nexthop, which select either the remote host-
name or the nexthop domain respectively. DNS names for SNI must be in
A-label (punycode) form. Invalid DNS names log a configuration error
warning and mail delivery is deferred.

Except when using a relayhost to forward all email, the only sensible
non-empty main.cf setting for this parameter is hostname. Other
non-empty values are only practical on a per-destination basis via the
servername attribute of the Postfix TLS policy table. When in doubt,
leave this parameter empty, and configure per-destination SNI as needed.

This feature is available in Postfix 3.4 and later.

smtp_tls_session_cache_database (default: empty)
Name of the file containing the optional Postfix SMTP client TLS session
cache. Specify a database type that supports enumeration, such as btree
or sdbm; there is no need to support concurrent access. The file is
created if it does not exist. The smtp(8) daemon does not use this para-
meter directly, rather the cache is implemented indirectly in the
tlsmgr(8) daemon. This means that per-smtp-instance master.cf overrides
of this parameter are not effective. Note that each of the cache data-
bases supported by tlsmgr(8) daemon: $smtpd_tls_session_cache_database,
$smtp_tls_session_cache_database (and with Postfix 2.3 and later
$lmtp_tls_session_cache_database), needs to be stored separately. It is
not at this time possible to store multiple caches in a single database.

Note: dbm databases are not suitable. TLS session objects are too large.

As of version 2.5, Postfix no longer uses root privileges when opening
this file. The file should now be stored under the Postfix-owned
data_directory. As a migration aid, an attempt to open the file under a
non-Postfix directory is redirected to the Postfix-owned data_directory,
and a warning is logged.

Example:

smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_scache

This feature is available in Postfix 2.2 and later.

smtp_tls_session_cache_timeout (default: 3600s)
The expiration time of Postfix SMTP client TLS session cache informa-
tion. A cache cleanup is performed periodically every $smtp_tls_ses-
sion_cache_timeout seconds. As with $smtp_tls_session_cache_database,
this parameter is implemented in the tlsmgr(8) daemon and therefore
per-smtp-instance master.cf overrides are not possible.

As of Postfix 2.11 this setting cannot exceed 100 days. If set <= 0,
session caching is disabled. If set to a positive value less than 2
minutes, the minimum value of 2 minutes is used instead.

This feature is available in Postfix 2.2 and later.

smtp_tls_trust_anchor_file (default: empty)
Zero or more PEM-format files with trust-anchor certificates and/or pub-
lic keys. If the parameter is not empty the root CAs in CAfile and CAp-
ath are no longer trusted. Rather, the Postfix SMTP client will only
trust certificate-chains signed by one of the trust-anchors contained in
the chosen files. The specified trust-anchor certificates and public
keys are not subject to expiration, and need not be (self-signed) root
CAs. They may, if desired, be intermediate certificates. Therefore,
these certificates also may be found "in the middle" of the trust chain
presented by the remote SMTP server, and any untrusted issuing parent
certificates will be ignored. Specify a list of pathnames separated by
comma or whitespace.

Whether specified in main.cf, or on a per-destination basis, the
trust-anchor PEM file must be accessible to the Postfix SMTP client in
the chroot jail if applicable. The trust-anchor file should contain
only certificates and public keys, no private key material, and must be
readable by the non-privileged $mail_owner user. This allows destina-
tions to be bound to a set of specific CAs or public keys without trust-
ing the same CAs for all destinations.

The main.cf parameter supports single-purpose Postfix installations that
send mail to a fixed set of SMTP peers. At most sites, if trust-anchor
files are used at all, they will be specified on a per-destination basis
via the "tafile" attribute of the "verify" and "secure" levels in
smtp_tls_policy_maps.

The underlying mechanism is in support of RFC 7672 (DANE TLSA), which
defines mechanisms for an SMTP client MTA to securely determine server
TLS certificates via DNS.

If you want your trust anchors to be public keys, with OpenSSL you can
extract a single PEM public key from a PEM X.509 file containing a sin-
gle certificate, as follows:

$ openssl x509 -in cert.pem -out ta-key.pem -noout -pubkey

This feature is available in Postfix 2.11 and later.

smtp_tls_verify_cert_match (default: hostname)
How the Postfix SMTP client verifies the server certificate peername for
the "verify" TLS security level. In a "verify" TLS policy table
($smtp_tls_policy_maps) entry the optional "match" attribute overrides
this main.cf setting.

This parameter specifies one or more patterns or strategies separated by
commas, whitespace or colons. In the policy table the only valid sepa-
rator is the colon character.

Patterns specify domain names, or domain name suffixes:

example.com
Match the example.com domain, i.e. one of the names in the server
certificate must be example.com. Upper and lower case distinc-
tions are ignored.

.example.com
Match subdomains of the example.com domain, i.e. match a name in
the server certificate that consists of a non-zero number of la-
bels followed by a .example.com suffix. Case distinctions are ig-
nored.

Strategies specify a transformation from the next-hop domain to the ex-
pected name in the server certificate:

nexthop
Match against the next-hop domain, which is either the recipient
domain, or the transport next-hop configured for the domain
stripped of any optional socket type prefix, enclosing square
brackets and trailing port. When MX lookups are not suppressed,
this is the original nexthop domain prior to the MX lookup, not
the result of the MX lookup. For LMTP delivery via UNIX-domain
sockets, the verified next-hop name is $myhostname. This strat-
egy is suitable for use with the "secure" policy. Case is ig-
nored.

dot-nexthop
As above, but match server certificate names that are subdomains
of the next-hop domain. Case is ignored.

hostname
Match against the hostname of the server, often obtained via an
unauthenticated DNS MX lookup. For LMTP delivery via UNIX-domain
sockets, the verified name is $myhostname. This matches the veri-
fication strategy of the "MUST" keyword in the obsolete
smtp_tls_per_site table, and is suitable for use with the "ver-
ify" security level. When the next-hop name is enclosed in square
brackets to suppress MX lookups, the "hostname" strategy is the
same as the "nexthop" strategy. Case is ignored.

Sample main.cf setting:

smtp_tls_verify_cert_match = hostname, nexthop, dot-nexthop

Sample policy table override:

example.com verify match=hostname:nexthop
.example.com verify match=example.com:.example.com:hostname

This feature is available in Postfix 2.3 and later.

smtp_tls_wrappermode (default: no)
Request that the Postfix SMTP client connects using the SUBMISSIONS
(formerly called SMTPS) protocol instead of using the STARTTLS command.

This mode requires "smtp_tls_security_level = encrypt" or stronger.

Example: deliver all remote mail via a provider's submissions service at
"mail.example.com".

/etc/postfix/main.cf:
# Client-side SUBMISSIONS requires "encrypt" or stronger.
smtp_tls_security_level = encrypt
smtp_tls_wrappermode = yes
# The [] suppress MX lookups.
relayhost = [mail.example.com]:submissions

More examples are in TLS_README, including examples for older Postfix
versions.

This feature is available in Postfix 3.0 and later.

smtp_tlsrpt_enable (default: no)
Enable support for RFC 8460 TLSRPT notifications. A mail receiving do-
main can publish a TLSRPT policy in DNS, to request periodic summaries
of successful and failed SMTP over TLS connections to their MX servers.
This feature requires that Postfix is built with a TLSRPT supporting li-
brary.

See TLSRPT_README for configuration examples and additional requirements
for MTA-STS smtp_tls_policy_maps plugins.

This feature is available in Postfix >= 3.10.

smtp_tlsrpt_skip_reused_handshakes (default: yes)
Do not report the TLSRPT status for TLS protocol handshakes that reuse a
previously-negotiated TLS session (there is no new information to re-
port). Report the TLSRPT status only for "new" TLS sessions. Set this to
"no" to log the TLSRPT status of all TLS handshakes, for example to
troubleshoot Postfix TLSRPT support.

Note: if an SMTP over TLS connection is reused, there is no second etc.
TLS handshake to report.

This feature is available in Postfix >= 3.10.

smtp_tlsrpt_socket_name (default: empty)
The pathname of a UNIX-domain datagram socket that is managed by a local
TLSRPT reporting service. This parameter must specify a pathname (ab-
solute, or relative to $queue_directory) when "smtp_tlsrpt_enable =
yes".

See TLSRPT_README for configuration examples and additional requirements
for MTA-STS smtp_tls_policy_maps plugins.

This feature is available in Postfix >= 3.10.

smtp_use_tls (default: no)
Opportunistic mode: use TLS when a remote SMTP server announces STARTTLS
support, otherwise send the mail in the clear. Beware: some SMTP servers
offer STARTTLS even if it is not configured. With Postfix < 2.3, if the
TLS handshake fails, and no other server is available, delivery is de-
ferred and mail stays in the queue. If this is a concern for you, use
the smtp_tls_per_site feature instead.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and
later use smtp_tls_security_level instead.

smtp_xforward_timeout (default: 300s)
The Postfix SMTP client time limit for sending the XFORWARD command, and
for receiving the remote SMTP server response.

This feature is available in Postfix 2.1 and later.

smtpd_authorized_verp_clients (default: $authorized_verp_clients)
What remote SMTP clients are allowed to specify the XVERP command. This
command requests that mail be delivered one recipient at a time with a
per recipient return address.

By default, no clients are allowed to specify XVERP.

This parameter was renamed with Postfix version 2.1. The default value
is backwards compatible with Postfix version 2.0.

Note: IP version 6 address information must be specified inside [] in
the smtpd_authorized_verp_clients value, and in files specified with
"/file/name". IP version 6 addresses contain the ":" character, and
would otherwise be confused with a "type:table" pattern.

smtpd_authorized_xclient_hosts (default: empty)
What remote SMTP clients are allowed to use the XCLIENT feature. This
command overrides remote SMTP client information that is used for access
control. Typical use is for SMTP-based content filters, fetchmail-like
programs, or SMTP server access rule testing. See the XCLIENT_README
document for details.

This feature is available in Postfix 2.1 and later.

By default, no clients are allowed to specify XCLIENT.

Note: IP version 6 address information must be specified inside [] in
the smtpd_authorized_xclient_hosts value, and in files specified with
"/file/name". IP version 6 addresses contain the ":" character, and
would otherwise be confused with a "type:table" pattern.

smtpd_authorized_xforward_hosts (default: empty)
What remote SMTP clients are allowed to use the XFORWARD feature. This
command forwards information that is used to improve logging after
SMTP-based content filters. See the XFORWARD_README document for de-
tails.

This feature is available in Postfix 2.1 and later.

By default, no clients are allowed to specify XFORWARD.

Note: IP version 6 address information must be specified inside [] in
the smtpd_authorized_xforward_hosts value, and in files specified with
"/file/name". IP version 6 addresses contain the ":" character, and
would otherwise be confused with a "type:table" pattern.

smtpd_banner (default: $myhostname ESMTP $mail_name)
The text that follows the 220 status code in the SMTP greeting banner.
Some people like to see the mail version advertised. By default, Postfix
shows no version.

You MUST specify $myhostname at the start of the text. This is required
by the SMTP protocol.

Example:

smtpd_banner = $myhostname ESMTP $mail_name ($mail_version)

smtpd_client_auth_rate_limit (default: 0)
The maximal number of AUTH commands that any client is allowed to send
to this service per time unit, regardless of whether or not Postfix ac-
tually accepts those commands. The time unit is specified with the
anvil_rate_time_unit configuration parameter.

By default, there is no limit on the number of AUTH commands that a
client may send.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be
used to regulate legitimate mail traffic.

This feature is available in Postfix 3.1 and later.

smtpd_client_connection_count_limit (default: 50)
How many simultaneous connections any client is allowed to make to this
service. By default, the limit is set to half the default process limit
value.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be
used to regulate legitimate mail traffic.

This feature is available in Postfix 2.2 and later.

smtpd_client_connection_rate_limit (default: 0)
The maximal number of connection attempts any client is allowed to make
to this service per time unit. The time unit is specified with the
anvil_rate_time_unit configuration parameter.

By default, a client can make as many connections per time unit as Post-
fix can accept.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be
used to regulate legitimate mail traffic.

This feature is available in Postfix 2.2 and later.

Example:

smtpd_client_connection_rate_limit = 1000

smtpd_client_event_limit_exceptions (default: $mynetworks)
Clients that are excluded from smtpd_client_*_count/rate_limit restric-
tions. See the mynetworks parameter description for the parameter value
syntax.

By default, clients in trusted networks are excluded. Specify a list of
network blocks, hostnames or .domain names (the initial dot causes the
domain to match any name below it).

Note: IP version 6 address information must be specified inside [] in
the smtpd_client_event_limit_exceptions value, and in files specified
with "/file/name". IP version 6 addresses contain the ":" character,
and would otherwise be confused with a "type:table" pattern.

Pattern matching of domain names is controlled by the presence or ab-
sence of "smtpd_client_event_limit_exceptions" in the parent_do-
main_matches_subdomains parameter value (Postfix 3.0 and later).

This feature is available in Postfix 2.2 and later.

smtpd_client_ipv4_prefix_length (default: 32)
Aggregate smtpd_client_*_count and smtpd_client_*_rate statistics by
IPv4 network blocks with the specified network prefix. Aggregation uses
fewer anvil(8) resources to maintain counters. By default, aggregation
is disabled for IPv4.

This feature is available in Postfix 3.8 and later.

smtpd_client_ipv6_prefix_length (default: 84)
Aggregate smtpd_client_*_count and smtpd_client_*_rate statistics by
IPv6 network blocks with the specified network prefix. Aggregation uses
fewer the anvil(8) resources to maintain counters. By default, aggrega-
tion is enabled for IPv6.

This feature is available in Postfix 3.8 and later.

smtpd_client_message_rate_limit (default: 0)
The maximal number of message delivery requests that any client is al-
lowed to make to this service per time unit, regardless of whether or
not Postfix actually accepts those messages. The time unit is specified
with the anvil_rate_time_unit configuration parameter.

By default, a client can send as many message delivery requests per time
unit as Postfix can accept.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be
used to regulate legitimate mail traffic.

This feature is available in Postfix 2.2 and later.

Example:

smtpd_client_message_rate_limit = 1000

smtpd_client_new_tls_session_rate_limit (default: 0)
The maximal number of new (i.e., uncached) TLS sessions that a remote
SMTP client is allowed to negotiate with this service per time unit.
The time unit is specified with the anvil_rate_time_unit configuration
parameter.

By default, a remote SMTP client can negotiate as many new TLS sessions
per time unit as Postfix can accept.

To disable this feature, specify a limit of 0. Otherwise, specify a
limit that is at least the per-client concurrent session limit, or else
legitimate client sessions may be rejected.

WARNING: The purpose of this feature is to limit abuse. It must not be
used to regulate legitimate mail traffic.

This feature is available in Postfix 2.3 and later.

Example:

smtpd_client_new_tls_session_rate_limit = 100

smtpd_client_port_logging (default: no)
Enable logging of the remote SMTP client port in addition to the host-
name and IP address. The logging format is "host[address]:port".

This feature is available in Postfix 2.5 and later.

smtpd_client_recipient_rate_limit (default: 0)
The maximal number of recipient addresses that any client is allowed to
send to this service per time unit, regardless of whether or not Postfix
actually accepts those recipients. The time unit is specified with the
anvil_rate_time_unit configuration parameter.

By default, a client can send as many recipient addresses per time unit
as Postfix can accept.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be
used to regulate legitimate mail traffic.

This feature is available in Postfix 2.2 and later.

Example:

smtpd_client_recipient_rate_limit = 1000

smtpd_client_restrictions (default: empty)
Optional restrictions that the Postfix SMTP server applies in the con-
text of a client connection request. See SMTPD_ACCESS_README, section
"Delayed evaluation of SMTP access restriction lists" for a discussion
of evaluation context and time.

The default is to allow all connection requests.

Specify a list of restrictions, separated by commas and/or whitespace.
Continue long lines by starting the next line with whitespace. Restric-
tions are applied in the order as specified; the first restriction that
matches wins.

The following restrictions are specific to client hostname or client
network address information.

check_ccert_access type:table
By default use the remote SMTP client certificate fingerprint or
the public key fingerprint (Postfix 2.9 and later) as the lookup
key for the specified access(5) database; with Postfix version
2.2, also require that the remote SMTP client certificate is ver-
ified successfully. The fingerprint digest algorithm is config-
urable via the smtpd_tls_fingerprint_digest parameter (hard-coded
as md5 prior to Postfix version 2.5). This feature requires
"smtpd_tls_ask_ccert = yes" and is available with Postfix version
2.2 and later.
The default algorithm is sha256 with Postfix >= 3.6 and the com-
patibility_level set to 3.6 or higher. With Postfix <= 3.5, the
default algorithm is md5. The best-practice algorithm is now
sha256. Recent advances in hash function cryptanalysis have led
to md5 and sha1 being deprecated in favor of sha256. However, as
long as there are no known "second pre-image" attacks against the
older algorithms, their use in this context, though not recom-
mended, is still likely safe.
Alternatively, check_ccert_access accepts an explicit search or-
der (Postfix 3.5 and later). The default search order as de-
scribed above corresponds with:
check_ccert_access { type:table, { search_order = cert_finger-
print, pubkey_fingerprint } }
The commas are optional.

check_client_access type:table
Search the specified access database for the client hostname or
IP address. See the access(5) manual page for details.

check_client_a_access type:table
Search the specified access(5) database for the IP addresses for
the client hostname, and execute the corresponding action. Note:
a result of "OK" is not allowed for safety reasons. Instead, use
DUNNO in order to exclude specific hosts from denylists. This
feature is available in Postfix 3.0 and later.

check_client_mx_access type:table
Search the specified access(5) database for the MX hosts for the
client hostname, and execute the corresponding action. If no MX
record is found, look up A or AAAA records, just like the Postfix
SMTP client would. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific
hosts from denylists. This feature is available in Postfix 2.7
and later.

check_client_ns_access type:table
Search the specified access(5) database for the DNS servers for
the client hostname, and execute the corresponding action. Note:
a result of "OK" is not allowed for safety reasons. Instead, use
DUNNO in order to exclude specific hosts from denylists. This
feature is available in Postfix 2.7 and later.

check_reverse_client_hostname_access type:table
Search the specified access database for the unverified reverse
client hostname or IP address. See the access(5) manual page for
details. Note: a result of "OK" is not allowed for safety rea-
sons. Instead, use DUNNO in order to exclude specific hosts from
denylists. This feature is available in Postfix 2.6 and later.

check_reverse_client_hostname_a_access type:table
Search the specified access(5) database for the IP addresses for
the unverified reverse client hostname, and execute the corre-
sponding action. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific
hosts from denylists. This feature is available in Postfix 3.0
and later.

check_reverse_client_hostname_mx_access type:table
Search the specified access(5) database for the MX hosts for the
unverified reverse client hostname, and execute the corresponding
action. If no MX record is found, look up A or AAAA records,
just like the Postfix SMTP client would. Note: a result of "OK"
is not allowed for safety reasons. Instead, use DUNNO in order
to exclude specific hosts from denylists. This feature is avail-
able in Postfix 2.7 and later.

check_reverse_client_hostname_ns_access type:table
Search the specified access(5) database for the DNS servers for
the unverified reverse client hostname, and execute the corre-
sponding action. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific
hosts from denylists. This feature is available in Postfix 2.7
and later.

check_sasl_access type:table
Use the remote SMTP client SASL user name as the lookup key for
the specified access(5) database. The lookup key has the form
"username@domainname" when the smtpd_sasl_local_domain parameter
value is non-empty. Unlike the check_client_access feature,
check_sasl_access does not perform matches of parent domains or
IP subnet ranges. This feature is available with Postfix version
2.11 and later.

permit_inet_interfaces
Permit the request when the client IP address matches $inet_in-
terfaces.

permit_mynetworks
Permit the request when the client IP address matches any network
or network address listed in $mynetworks.

permit_sasl_authenticated
Permit the request when the client is successfully authenticated
via the RFC 4954 (AUTH) protocol.

permit_tls_all_clientcerts
Permit the request when the remote SMTP client certificate is
verified successfully. This option must be used only if a spe-
cial CA issues the certificates and only this CA is listed as a
trusted CA. Otherwise, clients with a third-party certificate
would also be allowed to relay. Specify "tls_append_default_CA =
no" when the trusted CA is specified with smtpd_tls_CAfile or
smtpd_tls_CApath, to prevent Postfix from appending the sys-
tem-supplied default CAs. This feature requires
"smtpd_tls_ask_ccert = yes" and is available with Postfix version
2.2 and later.

permit_tls_clientcerts
Permit the request when the remote SMTP client certificate fin-
gerprint or public key fingerprint (Postfix 2.9 and later) is
listed in $relay_clientcerts. The fingerprint digest algorithm
is configurable via the smtpd_tls_fingerprint_digest parameter
(hard-coded as md5 prior to Postfix version 2.5). This feature
requires "smtpd_tls_ask_ccert = yes" and is available with Post-
fix version 2.2 and later.
The default algorithm is sha256 with Postfix >= 3.6 and the com-
patibility_level set to 3.6 or higher. With Postfix <= 3.5, the
default algorithm is md5. The best-practice algorithm is now
sha256. Recent advances in hash function cryptanalysis have led
to md5 and sha1 being deprecated in favor of sha256. However, as
long as there are no known "second pre-image" attacks against the
older algorithms, their use in this context, though not recom-
mended, is still likely safe.

reject_rbl_client rbl_domain=d.d.d.d
Reject the request when the reversed client network address is
listed with the A record "d.d.d.d" under rbl_domain (Postfix ver-
sion 2.1 and later only). Each "d" is a number, or a pattern in-
side "[]" that contains one or more ";"-separated numbers or num-
ber..number ranges (Postfix version 2.8 and later). If no
"=d.d.d.d" is specified, reject the request when the reversed
client network address is listed with any A record under rbl_do-
main.
The maps_rbl_reject_code parameter specifies the response code
for rejected requests (default: 554), the default_rbl_reply pa-
rameter specifies the default server reply, and the rbl_re-
ply_maps parameter specifies tables with server replies indexed
by rbl_domain. This feature is available in Postfix 2.0 and
later.

permit_dnswl_client dnswl_domain=d.d.d.d
Accept the request when the reversed client network address is
listed with the A record "d.d.d.d" under dnswl_domain. Each "d"
is a number, or a pattern inside "[]" that contains one or more
";"-separated numbers or number..number ranges. If no "=d.d.d.d"
is specified, accept the request when the reversed client network
address is listed with any A record under dnswl_domain.
For safety, permit_dnswl_client is silently ignored when it would
override reject_unauth_destination. The result is DEFER_IF_RE-
JECT when allowlist lookup fails. This feature is available in
Postfix 2.8 and later.

reject_rhsbl_client rbl_domain=d.d.d.d
Reject the request when the client hostname is listed with the A
record "d.d.d.d" under rbl_domain (Postfix version 2.1 and later
only). Each "d" is a number, or a pattern inside "[]" that con-
tains one or more ";"-separated numbers or number..number ranges
(Postfix version 2.8 and later). If no "=d.d.d.d" is specified,
reject the request when the client hostname is listed with any A
record under rbl_domain. See the reject_rbl_client description
above for additional RBL related configuration parameters. This
feature is available in Postfix 2.0 and later; with Postfix ver-
sion 2.8 and later, reject_rhsbl_reverse_client will usually pro-
duce better results.

permit_rhswl_client rhswl_domain=d.d.d.d
Accept the request when the client hostname is listed with the A
record "d.d.d.d" under rhswl_domain. Each "d" is a number, or a
pattern inside "[]" that contains one or more ";"-separated num-
bers or number..number ranges. If no "=d.d.d.d" is specified, ac-
cept the request when the client hostname is listed with any A
record under rhswl_domain.
Caution: client name allowlisting is fragile, since the client
name lookup can fail due to temporary outages. Client name al-
lowlisting should be used only to reduce false positives in e.g.
DNS-based blocklists, and not for making access rule exceptions.
For safety, permit_rhswl_client is silently ignored when it would
override reject_unauth_destination. The result is DEFER_IF_RE-
JECT when allowlist lookup fails. This feature is available in
Postfix 2.8 and later.

reject_rhsbl_reverse_client rbl_domain=d.d.d.d
Reject the request when the unverified reverse client hostname is
listed with the A record "d.d.d.d" under rbl_domain. Each "d" is
a number, or a pattern inside "[]" that contains one or more
";"-separated numbers or number..number ranges. If no "=d.d.d.d"
is specified, reject the request when the unverified reverse
client hostname is listed with any A record under rbl_domain. See
the reject_rbl_client description above for additional RBL re-
lated configuration parameters. This feature is available in
Postfix 2.8 and later.

reject_unknown_client_hostname (with Postfix < 2.3: reject_un-
known_client)
Reject the request when 1) the client IP address->name mapping
fails, or 2) the name->address mapping fails, or 3) the name->ad-
dress mapping does not match the client IP address.
This is a stronger restriction than the reject_unknown_re-
verse_client_hostname feature, which triggers only under condi-
tion 1) above.
The unknown_client_reject_code parameter specifies the response
code for rejected requests (default: 450). The reply is always
450 in case the address->name or name->address lookup failed due
to a temporary problem.

reject_unknown_reverse_client_hostname
Reject the request when the client IP address has no ad-
dress->name mapping.
This is a weaker restriction than the reject_unknown_client_host-
name feature, which requires not only that the address->name and
name->address mappings exist, but also that the two mappings re-
produce the client IP address.
The unknown_client_reject_code parameter specifies the response
code for rejected requests (default: 450). The reply is always
450 in case the address->name lookup failed due to a temporary
problem.
This feature is available in Postfix 2.3 and later.

In addition, you can use any of the following generic restrictions.
These restrictions are applicable in any SMTP command context.

check_policy_service servername
Query the specified policy server. See the SMTPD_POLICY_README
document for details. This feature is available in Postfix 2.1
and later.

defer Defer the request. The client is told to try again later. This
restriction is useful at the end of a restriction list, to make
the default policy explicit.
The defer_code parameter specifies the SMTP server reply code
(default: 450).

defer_if_permit
Defer the request if some later restriction would result in an
explicit or implicit PERMIT action. This is useful when a
denylisting feature fails due to a temporary problem. This fea-
ture is available in Postfix version 2.1 and later.

defer_if_reject
Defer the request if some later restriction would result in a RE-
JECT action. This is useful when an allowlisting feature fails
due to a temporary problem. This feature is available in Postfix
version 2.1 and later.

permit Permit the request. This restriction is useful at the end of a
restriction list, to make the default policy explicit.

reject_multi_recipient_bounce
Reject the request when the envelope sender is the null address,
and the message has multiple envelope recipients. This usage has
rare but legitimate applications: under certain conditions,
multi-recipient mail that was posted with the DSN option NO-
TIFY=NEVER may be forwarded with the null sender address.
Note: this restriction can only work reliably when used in
smtpd_data_restrictions or smtpd_end_of_data_restrictions, be-
cause the total number of recipients is not known at an earlier
stage of the SMTP conversation. Use at the RCPT stage will only
reject the second etc. recipient.
The multi_recipient_bounce_reject_code parameter specifies the
response code for rejected requests (default: 550). This fea-
ture is available in Postfix 2.1 and later.

reject_plaintext_session
Reject the request when the connection is not encrypted. This re-
striction should not be used before the client has had a chance
to negotiate encryption with the AUTH or STARTTLS commands.
The plaintext_reject_code parameter specifies the response code
for rejected requests (default: 450). This feature is available
in Postfix 2.3 and later.

reject_unauth_pipelining
Reject the request when the client sends SMTP commands ahead of
time where it is not allowed, or when the client sends SMTP com-
mands ahead of time without knowing that Postfix actually sup-
ports ESMTP command pipelining. This stops mail from bulk mail
software that improperly uses ESMTP command pipelining in order
to speed up deliveries.
With Postfix 2.6 and later, the SMTP server sets a per-session
flag whenever it detects illegal pipelining, including pipelined
HELO or EHLO commands. The reject_unauth_pipelining feature sim-
ply tests whether the flag was set at any point in time during
the session.
With older Postfix versions, reject_unauth_pipelining checks the
current status of the input read queue, and its usage is not rec-
ommended in contexts other than smtpd_data_restrictions.

reject Reject the request. This restriction is useful at the end of a
restriction list, to make the default policy explicit. The re-
ject_code configuration parameter specifies the response code for
rejected requests (default: 554).

sleep seconds
Pause for the specified number of seconds and proceed with the
next restriction in the list, if any. This may stop zombie mail
when used as:
/etc/postfix/main.cf:
smtpd_client_restrictions =
sleep 1, reject_unauth_pipelining
smtpd_delay_reject = no
This feature is available in Postfix 2.3.

warn_if_reject
A safety net for testing. When "warn_if_reject" is placed before
a reject-type restriction, access table query, or check_pol-
icy_service query, this logs a "reject_warning" message instead
of rejecting a request (when a reject-type restriction fails due
to a temporary error, this logs a "reject_warning" message for
any implicit "defer_if_permit" actions that would normally pre-
vent mail from being accepted by some later access restriction).
This feature has no effect on defer_if_reject restrictions.

Other restrictions that are valid in this context:

• SMTP command specific restrictions that are described under the
smtpd_helo_restrictions, smtpd_sender_restrictions or smtpd_re-
cipient_restrictions parameters. When helo, sender or recipient
restrictions are listed under smtpd_client_restrictions, they
have effect only with "smtpd_delay_reject = yes", so that
$smtpd_client_restrictions is evaluated at the time of the RCPT
TO command.

Example:

smtpd_client_restrictions = permit_mynetworks, reject_unknown_client_hostname

smtpd_command_filter (default: empty)
A mechanism to transform commands from remote SMTP clients. This is a
last-resort tool to work around client commands that break interoper-
ability with the Postfix SMTP server. Other uses involve fault injec-
tion to test Postfix's handling of invalid commands.

Specify the name of a "type:table" lookup table. The search string is
the SMTP command as received from the remote SMTP client, except that
initial whitespace and the trailing <CR><LF> are removed. The result
value is executed by the Postfix SMTP server.

There is no need to use smtpd_command_filter for the following cases:

• Use "resolve_numeric_domain = yes" to accept "user@ipaddress".

• Postfix already accepts the correct form "user@[ipaddress]". Use
virtual_alias_maps or canonical_maps to translate these into do-
main names if necessary.

• Use "strict_rfc821_envelopes = no" to accept "RCPT TO:<User Name
<user@example.com>>". Postfix will ignore the "User Name" part
and deliver to the <user@example.com> address.

Examples of problems that can be solved with the smtpd_command_filter
feature:

/etc/postfix/main.cf:
smtpd_command_filter = pcre:/etc/postfix/command_filter

/etc/postfix/command_filter:
# Work around clients that send malformed HELO commands.
/^HELO\s*$/ HELO domain.invalid

# Work around clients that send empty lines.
/^\s*$/ NOOP

# Work around clients that send RCPT TO:<'user@domain'>.
# WARNING: do not lose the parameters that follow the address.
/^(RCPT\s+TO:\s*<)'([^[:space:]]+)'(>.*)/ $1$2$3

# Append XVERP to MAIL FROM commands to request VERP-style delivery.
# See VERP_README for more information on how to use Postfix VERP.
/^(MAIL\s+FROM:\s*<listname@example\.com>.*)/ $1 XVERP

# Bounce-never mail sink. Use notify_classes=bounce,resource,software
# to send bounced mail to the postmaster (with message body removed).
/^(RCPT\s+TO:\s*<.*>.*)\s+NOTIFY=\S+(.*)/ $1 NOTIFY=NEVER$2
/^(RCPT\s+TO:.*)/ $1 NOTIFY=NEVER

This feature is available in Postfix 2.7.

smtpd_data_restrictions (default: empty)
Optional access restrictions that the Postfix SMTP server applies in the
context of the SMTP DATA command. See SMTPD_ACCESS_README, section "De-
layed evaluation of SMTP access restriction lists" for a discussion of
evaluation context and time.

This feature is available in Postfix 2.0 and later.

The following restrictions are valid in this context:

• Generic restrictions that can be used in any SMTP command con-
text, described under smtpd_client_restrictions.

• SMTP command specific restrictions described under
smtpd_client_restrictions, smtpd_helo_restrictions,
smtpd_sender_restrictions or smtpd_recipient_restrictions.

• However, no recipient information is available in the case of
multi-recipient mail. Acting on only one recipient would be mis-
leading, because any decision will affect all recipients equally.
Acting on all recipients would require a possibly very large
amount of memory, and would also be misleading for the reasons
mentioned before.

Examples:

smtpd_data_restrictions = reject_unauth_pipelining
smtpd_data_restrictions = reject_multi_recipient_bounce

smtpd_delay_open_until_valid_rcpt (default: yes)
Postpone the start of an SMTP mail transaction until a valid RCPT TO
command is received. Specify "no" to create a mail transaction as soon
as the Postfix SMTP server receives a valid MAIL FROM command.

With sites that reject lots of mail, the default setting reduces the use
of disk, CPU and memory resources. The downside is that rejected recipi-
ents are logged with NOQUEUE instead of a mail transaction ID (also
known as a queue ID).

This feature is available in Postfix 2.3 and later.

smtpd_delay_reject (default: yes)
Wait until the RCPT TO command before evaluating $smtpd_client_restric-
tions, $smtpd_helo_restrictions and $smtpd_sender_restrictions, or wait
until the ETRN command before evaluating $smtpd_client_restrictions and
$smtpd_helo_restrictions.

This feature is turned on by default because some clients apparently
mis-behave when the Postfix SMTP server rejects commands before RCPT TO.

The default setting has one major benefit: it allows Postfix to log re-
cipient address information when rejecting a client name/address or
sender address, so that it is possible to find out whose mail is being
rejected.

smtpd_discard_ehlo_keyword_address_maps (default: empty)
Lookup tables, indexed by the remote SMTP client address, with case in-
sensitive lists of EHLO keywords (pipelining, starttls, auth, etc.) that
the Postfix SMTP server will not send in the EHLO response to a remote
SMTP client. See smtpd_discard_ehlo_keywords for details. The tables
are not searched by hostname for robustness reasons.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

This feature is available in Postfix 2.2 and later.

smtpd_discard_ehlo_keywords (default: empty)
A case insensitive list of EHLO keywords (pipelining, starttls, auth,
etc.) that the Postfix SMTP server will not send in the EHLO response to
a remote SMTP client.

This feature is available in Postfix 2.2 and later.

Notes:

• Specify the silent-discard pseudo keyword to prevent this action
from being logged.

• Use the smtpd_discard_ehlo_keyword_address_maps feature to dis-
card EHLO keywords selectively.

smtpd_dns_reply_filter (default: empty)
Optional filter for Postfix SMTP server DNS lookup results. See
smtp_dns_reply_filter for details including an example.

This feature is available in Postfix 3.0 and later.

smtpd_end_of_data_restrictions (default: empty)
Optional access restrictions that the Postfix SMTP server applies in the
context of the SMTP END-OF-DATA command. See SMTPD_ACCESS_README, sec-
tion "Delayed evaluation of SMTP access restriction lists" for a discus-
sion of evaluation context and time.

This feature is available in Postfix 2.2 and later.

See smtpd_data_restrictions for details and limitations.

smtpd_enforce_tls (default: no)
Mandatory TLS: announce STARTTLS support to remote SMTP clients, and re-
ject all plaintext commands except HELO, EHLO, XCLIENT, STARTTLS, NOOP,
QUIT, and (Postfix >= 3.9) HELP. According to RFC 2487 this MUST NOT be
applied in case of a publicly-referenced SMTP server. Instead, this
should be used on dedicated servers, for example submission (port 587).
This option is therefore off by default.

Note 1: "smtpd_enforce_tls = yes" implies "smtpd_tls_auth_only = yes".

Note 2: when invoked via "sendmail -bs", Postfix will never offer START-
TLS due to insufficient privileges to access the server private key.
This is intended behavior.

This feature is deprecated as of Postfix 3.9. Specify smtpd_tls_secu-
rity_level instead.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and
later use smtpd_tls_security_level instead.

smtpd_error_sleep_time (default: 1s)
With Postfix version 2.1 and later: the SMTP server response delay after
a client has made more than $smtpd_soft_error_limit errors, and fewer
than $smtpd_hard_error_limit errors, without delivering mail.

With Postfix version 2.0 and earlier: the SMTP server delay before send-
ing a reject (4xx or 5xx) response, when the client has made fewer than
$smtpd_soft_error_limit errors without delivering mail. When the client
has made $smtpd_soft_error_limit or more errors, delay all responses
with the larger of (number of errors) seconds or $smtpd_er-
ror_sleep_time.

smtpd_etrn_restrictions (default: empty)
Optional restrictions that the Postfix SMTP server applies in the con-
text of a client ETRN command. See SMTPD_ACCESS_README, section "De-
layed evaluation of SMTP access restriction lists" for a discussion of
evaluation context and time.

The Postfix ETRN implementation accepts only destinations that are eli-
gible for the Postfix "fast flush" service. See the ETRN_README file for
details.

The following restrictions are specific to the domain name information
received with the ETRN command.

check_etrn_access type:table
Search the specified access database for the ETRN domain name.
See the access(5) manual page for details.

Other restrictions that are valid in this context:

• Generic restrictions that can be used in any SMTP command con-
text, described under smtpd_client_restrictions.

• SMTP command specific restrictions described under
smtpd_client_restrictions and smtpd_helo_restrictions.

Example:

smtpd_etrn_restrictions = permit_mynetworks, reject

smtpd_expansion_filter (default: see postconf -d output)
What characters are allowed in $name expansions of RBL reply templates.
Characters not in the allowed set are replaced by "_". Use C like es-
capes to specify special characters such as whitespace.

The smtpd_expansion_filter value is not subject to Postfix configuration
parameter $name expansion.

This feature is available in Postfix 2.0 and later.

smtpd_forbid_bare_newline (default: Postfix >= 3.9: normalize)
Reject or restrict input lines from an SMTP client that end in <LF> in-
stead of the standard <CR><LF>. Such line endings are commonly allowed
with UNIX-based SMTP servers, but they violate RFC 5321, and allowing
such line endings can make a server vulnerable to SMTP smuggling.

Specify one of the following values (case does not matter):

normalize (default for Postfix >= 3.9)
Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>.
Otherwise, allow command or message content lines ending in the
non-standard <LF>, and process them as if the client sent the
standard <CR><LF>.
This maintains compatibility with many legitimate SMTP client ap-
plications that send a mix of standard and non-standard line end-
ings, but will fail to receive email from client implementations
that do not terminate DATA content with the standard End-of-DATA
sequence <CR><LF>.<CR><LF>.
Such clients can be excluded with smtpd_forbid_bare_newline_ex-
clusions.

note Same as "normalize", but also notes in the log whether the Post-
fix SMTP server received any lines with "bare <LF>". The informa-
tion is formatted as "disconnect from name[address] ...
notes=bare_lf". The notes value is expected to become a list of
comma-separated names.
This feature is available in Postfix 3.9 and later.

yes Compatibility alias for normalize.

reject Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>. Re-
ject a command or message content when a line contains bare <LF>,
log a "bare <LF> received" error, and reply with the SMTP status
code in $smtpd_forbid_bare_newline_reject_code.
This will reject email from SMTP clients that send any non-stan-
dard line endings such as web applications, netcat, or load bal-
ancer health checks.
This will also reject email from services that use BDAT to send
MIME text containing a bare newline (RFC 3030 Section 3 requires
canonical MIME format for text message types, defined in RFC 2045
Sections 2.7 and 2.8).
Such clients can be excluded with smtpd_forbid_bare_newline_ex-
clusions (or, in the case of BDAT violations, BDAT can be selec-
tively disabled with smtpd_discard_ehlo_keyword_address_maps, or
globally disabled with smtpd_discard_ehlo_keywords).

no (default for Postfix < 3.9)
Do not require the standard End-of-DATA sequence
<CR><LF>.<CR><LF>. Always process a bare <LF> as if the client
sent <CR><LF>. This option is fully backwards compatible, but is
not recommended for an Internet-facing SMTP server, because it is
vulnerable to SMTP smuggling.

Recommended settings:

# Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>.
# Otherwise, allow bare <LF> and process it as if the client sent
# <CR><LF>.
#
# This maintains compatibility with many legitimate SMTP client
# applications that send a mix of standard and non-standard line
# endings, but will fail to receive email from client implementations
# that do not terminate DATA content with the standard End-of-DATA
# sequence <CR><LF>.<CR><LF>.
#
# Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
# The example below allowlists SMTP clients in trusted networks.
#
smtpd_forbid_bare_newline = normalize
smtpd_forbid_bare_newline_exclusions = $mynetworks

Alternative:

# Reject input lines that contain <LF> and log a "bare <LF> received"
# error. Require that input lines end in <CR><LF>, and require the
# standard End-of-DATA sequence <CR><LF>.<CR><LF>.
#
# This will reject email from SMTP clients that send any non-standard
# line endings such as web applications, netcat, or load balancer
# health checks.
#
# This will also reject email from services that use BDAT to send
# MIME text containing a bare newline (RFC 3030 Section 3 requires
# canonical MIME format for text message types, defined in RFC 2045
# Sections 2.7 and 2.8).
#
# Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
# The example below allowlists SMTP clients in trusted networks.
#
smtpd_forbid_bare_newline = reject
smtpd_forbid_bare_newline_exclusions = $mynetworks
#
# Alternatively, in the case of BDAT violations, BDAT can be selectively
# disabled with smtpd_discard_ehlo_keyword_address_maps, or globally
# disabled with smtpd_discard_ehlo_keywords.
#
# smtpd_discard_ehlo_keyword_address_maps = cidr:/path/to/file
# /path/to/file:
# 10.0.0.0/24 chunking, silent-discard
# smtpd_discard_ehlo_keywords = chunking, silent-discard

This feature with settings yes and no is available in Postfix 3.8.4,
3.7.9, 3.6.13, and 3.5.23. Additionally, the settings reject, and nor-
malize are available with Postfix >= 3.9, 3.8.5, 3.7.10, 3.6.14, and
3.5.24.

smtpd_forbid_bare_newline_exclusions (default: $mynetworks)
Exclude the specified clients from smtpd_forbid_bare_newline enforce-
ment. This setting uses the same syntax and parent-domain matching be-
havior as mynetworks.

This feature is available in Postfix >= 3.9, 3.8.4, 3.7.9, 3.6.13, and
3.5.23.

smtpd_forbid_bare_newline_reject_code (default: 550)
The numerical Postfix SMTP server response code when rejecting a request
with "smtpd_forbid_bare_newline = reject". Specify a 5XX status code
(521 to disconnect).

This feature is available in Postfix >= 3.9, 3.8.5, 3.7.10, 3.6.14, and
3.5.24.

smtpd_forbid_unauth_pipelining (default: Postfix >= 3.9: yes)
Disconnect remote SMTP clients that violate RFC 2920 (or 5321) command
pipelining constraints. The server replies with "554 5.5.0 Error: SMTP
protocol synchronization" and logs the unexpected remote SMTP client in-
put. This feature is enabled by default with Postfix >= 3.9. Specify
"smtpd_forbid_unauth_pipelining = no" to disable.

This feature is available in Postfix >= 3.9, 3.8.1, 3.7.6, 3.6.10, and
3.5.20.

smtpd_forbidden_commands (default: CONNECT GET POST regexp:{{/^[^A-Z]/ Bogus}})

List of commands that cause the Postfix SMTP server to immediately ter-
minate the session with a 221 code. This can be used to disconnect
clients that obviously attempt to abuse the system. In addition to the
commands listed in this parameter, commands that follow the "Label:"
format of message headers will also cause a disconnect. With Postfix
versions 3.6 and earlier, the default value is "CONNECT GET POST".

This feature is available in Postfix 2.2 and later.

Support for inline regular expressions was added in Postfix version 3.7.
See regexp_table(5) for a description of the syntax and features.

smtpd_hard_error_limit (default: normal: 20, overload: 1)
The maximal number of errors a remote SMTP client is allowed to make
without delivering mail. The Postfix SMTP server disconnects when the
limit is reached. Normally the default limit is 20, but it changes under
overload to just 1. With Postfix 2.5 and earlier, the SMTP server always
allows up to 20 errors by default. Valid values are greater than zero.

smtpd_helo_required (default: no)
Require that a remote SMTP client introduces itself with the HELO or
EHLO command before sending the MAIL command or other commands that re-
quire EHLO negotiation.

Example:

smtpd_helo_required = yes

smtpd_helo_restrictions (default: empty)
Optional restrictions that the Postfix SMTP server applies in the con-
text of a client HELO command. See SMTPD_ACCESS_README, section "De-
layed evaluation of SMTP access restriction lists" for a discussion of
evaluation context and time.

The default is to permit everything.

Note: specify "smtpd_helo_required = yes" to fully enforce this restric-
tion (without "smtpd_helo_required = yes", a client can simply skip
smtpd_helo_restrictions by not sending HELO or EHLO).

The following restrictions are specific to the hostname information re-
ceived with the HELO or EHLO command.

check_helo_access type:table
Search the specified access(5) database for the HELO or EHLO
hostname, and execute the corresponding action. Note: specify
"smtpd_helo_required = yes" to fully enforce this restriction
(without "smtpd_helo_required = yes", a client can simply skip
check_helo_access by not sending HELO or EHLO).

check_helo_a_access type:table
Search the specified access(5) database for the IP addresses for
the HELO or EHLO hostname, and execute the corresponding action.
Note 1: a result of "OK" is not allowed for safety reasons. In-
stead, use DUNNO in order to exclude specific hosts from
denylists. Note 2: specify "smtpd_helo_required = yes" to fully
enforce this restriction (without "smtpd_helo_required = yes", a
client can simply skip check_helo_a_access by not sending HELO or
EHLO). This feature is available in Postfix 3.0 and later.

check_helo_mx_access type:table
Search the specified access(5) database for the MX hosts for the
HELO or EHLO hostname, and execute the corresponding action. If
no MX record is found, look up A or AAAA records, just like the
Postfix SMTP client would. Note 1: a result of "OK" is not al-
lowed for safety reasons. Instead, use DUNNO in order to exclude
specific hosts from denylists. Note 2: specify "smtpd_helo_re-
quired = yes" to fully enforce this restriction (without
"smtpd_helo_required = yes", a client can simply skip
check_helo_mx_access by not sending HELO or EHLO). This feature
is available in Postfix 2.1 and later.

check_helo_ns_access type:table
Search the specified access(5) database for the DNS servers for
the HELO or EHLO hostname, and execute the corresponding action.
Note 1: a result of "OK" is not allowed for safety reasons. In-
stead, use DUNNO in order to exclude specific hosts from
denylists. Note 2: specify "smtpd_helo_required = yes" to fully
enforce this restriction (without "smtpd_helo_required = yes", a
client can simply skip check_helo_ns_access by not sending HELO
or EHLO). This feature is available in Postfix 2.1 and later.

reject_invalid_helo_hostname (with Postfix < 2.3: reject_invalid_host-
name)
Reject the request when the HELO or EHLO hostname is malformed.
Note: specify "smtpd_helo_required = yes" to fully enforce this
restriction (without "smtpd_helo_required = yes", a client can
simply skip reject_invalid_helo_hostname by not sending HELO or
EHLO).
The invalid_hostname_reject_code specifies the response code for
rejected requests (default: 501).

reject_non_fqdn_helo_hostname (with Postfix < 2.3: reject_non_fqdn_host-
name)
Reject the request when the HELO or EHLO hostname is not in
fully-qualified domain or address literal form, as required by
the RFC. Note: specify "smtpd_helo_required = yes" to fully en-
force this restriction (without "smtpd_helo_required = yes", a
client can simply skip reject_non_fqdn_helo_hostname by not send-
ing HELO or EHLO).
The non_fqdn_reject_code parameter specifies the response code
for rejected requests (default: 504).

reject_rhsbl_helo rbl_domain=d.d.d.d
Reject the request when the HELO or EHLO hostname is listed with
the A record "d.d.d.d" under rbl_domain (Postfix version 2.1 and
later only). Each "d" is a number, or a pattern inside "[]" that
contains one or more ";"-separated numbers or number..number
ranges (Postfix version 2.8 and later). If no "=d.d.d.d" is
specified, reject the request when the HELO or EHLO hostname is
listed with any A record under rbl_domain. See the re-
ject_rbl_client description for additional RBL related configura-
tion parameters. Note: specify "smtpd_helo_required = yes" to
fully enforce this restriction (without "smtpd_helo_required =
yes", a client can simply skip reject_rhsbl_helo by not sending
HELO or EHLO). This feature is available in Postfix 2.0 and
later.

reject_unknown_helo_hostname (with Postfix < 2.3: reject_unknown_host-
name)
Reject the request when the HELO or EHLO hostname has no DNS A or
MX record.
The reply is specified with the unknown_hostname_reject_code pa-
rameter (default: 450) or unknown_helo_hostname_tempfail_action
(default: defer_if_permit). See the respective parameter de-
scriptions for details.
Note: specify "smtpd_helo_required = yes" to fully enforce this
restriction (without "smtpd_helo_required = yes", a client can
simply skip reject_unknown_helo_hostname by not sending HELO or
EHLO).

Other restrictions that are valid in this context:

• Generic restrictions that can be used in any SMTP command con-
text, described under smtpd_client_restrictions.

• Client hostname or network address specific restrictions de-
scribed under smtpd_client_restrictions.

• SMTP command specific restrictions described under
smtpd_sender_restrictions or smtpd_recipient_restrictions. When
sender or recipient restrictions are listed under smtpd_helo_re-
strictions, they have effect only with "smtpd_delay_reject =
yes", so that $smtpd_helo_restrictions is evaluated at the time
of the RCPT TO command.

Examples:

smtpd_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname
smtpd_helo_restrictions = permit_mynetworks, reject_unknown_helo_hostname

smtpd_hide_client_session (default: no)
Do not include SMTP client session information in the Postfix SMTP
server's Received: message header.

• The default setting, "smtpd_hide_client_session = no", must be
used for the port 25 MTA service. It provides information that is
required by RFC 5321.

• The setting "smtpd_hide_client_session = yes" may be used for the
port 587 and 465 MUA services. This hides the SMTP client host-
name and IP address, TLS session details, SASL login details, and
SMTP protocol details.

Depending on the number of recipients, a redacted Received: header has
one of the following forms:

Received: by mail.example.com (Postfix) id postfix-queue-id
for <user@example.com>; Day, dd Mon yyyy hh:mm:ss tz-offset (zone)
Received: by mail.example.com (Postfix) id postfix-queue-id
Day, dd Mon yyyy hh:mm:ss tz-offset (zone)

The redacted form hides that a message was received with SMTP, and
therefore it does not need to provide the information required by RFC
5321. The form does still meet RFC 5322 requirements.

This feature is available in Postfix >= 3.10.

smtpd_history_flush_threshold (default: 100)
The maximal number of lines in the Postfix SMTP server command history
before it is flushed upon receipt of EHLO, RSET, or end of DATA.

smtpd_junk_command_limit (default: normal: 100, overload: 1)
The number of junk commands (NOOP, VRFY, ETRN or RSET) that a remote
SMTP client can send before the Postfix SMTP server starts to increment
the error counter with each junk command. The junk command count is re-
set after mail is delivered. See also the smtpd_error_sleep_time and
smtpd_soft_error_limit configuration parameters. Normally the default
limit is 100, but it changes under overload to just 1. With Postfix 2.5
and earlier, the SMTP server always allows up to 100 junk commands by
default.

smtpd_log_access_permit_actions (default: empty)
Enable logging of the named "permit" actions in SMTP server access lists
(by default, the SMTP server logs "reject" actions but not "permit" ac-
tions). This feature does not affect conditional actions such as "de-
fer_if_permit".

Specify a list of "permit" action names, "/file/name" or "type:table"
patterns, separated by commas and/or whitespace. The list is matched
left to right, and the search stops on the first match. A "/file/name"
pattern is replaced by its contents; a "type:table" lookup table is
matched when a name matches a lookup key (the lookup result is ignored).
Continue long lines by starting the next line with whitespace. Specify
"!pattern" to exclude a name from the list.

Examples:

/etc/postfix/main.cf:
# Log all "permit" actions.
smtpd_log_access_permit_actions = static:all

/etc/postfix/main.cf:
# Log "permit_dnswl_client" only.
smtpd_log_access_permit_actions = permit_dnswl_client

This feature is available in Postfix 2.10 and later.

smtpd_milter_maps (default: empty)
Lookup tables with Milter settings per remote SMTP client IP address.
The lookup result overrides the smtpd_milters setting, and has the same
syntax.

Note: lookup tables cannot return empty responses. Specify a lookup re-
sult of DISABLE (case does not matter) to indicate that Milter support
should be disabled.

Example to disable Milters for local clients:

/etc/postfix/main.cf:
smtpd_milter_maps = cidr:/etc/postfix/smtpd_milter_map
smtpd_milters = inet:host:port, { inet:host:port, ... }, ...

/etc/postfix/smtpd_milter_map:
# Disable Milters for local clients.
127.0.0.0/8 DISABLE
192.168.0.0/16 DISABLE
::/64 DISABLE
2001:db8::/32 DISABLE

This feature is available in Postfix 3.2 and later.

smtpd_milters (default: empty)
A list of Milter (mail filter) applications for new mail that arrives
via the Postfix smtpd(8) server. Specify space or comma as separator.
See the MILTER_README document for details.

This feature is available in Postfix 2.3 and later.

smtpd_min_data_rate (default: 500)
The minimum plaintext data transfer rate in bytes/second for DATA and
BDAT requests, when deadlines are enabled with smtpd_per_request_dead-
line. After a read operation transfers N plaintext message bytes (possi-
bly after TLS decryption), and after the DATA or BDAT request deadline
is decremented by the elapsed time of that read operation, the DATA or
BDAT request deadline is incremented by N/smtpd_min_data_rate seconds.
However, the deadline will never be incremented beyond the time limit
specified with smtpd_timeout.

This feature is available in Postfix 3.7 and later.

smtpd_noop_commands (default: empty)
List of commands that the Postfix SMTP server replies to with "250 Ok",
without doing any syntax checks and without changing state. This list
overrides any commands built into the Postfix SMTP server.

smtpd_null_access_lookup_key (default: <>)
The lookup key to be used in SMTP access(5) tables instead of the null
sender address.

smtpd_peername_lookup (default: yes)
Attempt to look up the remote SMTP client hostname, and verify that the
name matches the client IP address. A client name is set to "unknown"
when it cannot be looked up or verified, or when name lookup is dis-
abled. Turning off name lookup reduces delays due to DNS lookup and in-
creases the maximal inbound delivery rate.

This feature is available in Postfix 2.3 and later.

smtpd_per_record_deadline (default: normal: no, overload: yes)
Change the behavior of the smtpd_timeout and smtpd_starttls_timeout time
limits, from a time limit per read or write system call, to a time limit
to send or receive a complete record (an SMTP command line, SMTP re-
sponse line, SMTP message content line, or TLS protocol message). This
limits the impact from hostile peers that trickle data one byte at a
time.

This feature is available in Postfix 2.9-3.6. With older Postfix re-
leases, the behavior is as if this parameter is set to "no". Postfix 3.7
and later use smtpd_per_request_deadline.

smtpd_per_request_deadline (default: normal: no, overload: yes)
Change the behavior of the smtpd_timeout and smtpd_starttls_timeout time
limits, from a time limit per plaintext or TLS read or write call, to a
combined time limit for receiving a complete SMTP request and for send-
ing a complete SMTP response. The deadline limits only the time spent
waiting for plaintext or TLS read or write calls, not time spent else-
where. The per-request deadline limits the impact from hostile peers
that trickle data one byte at a time.

See smtpd_min_data_rate for how the per-request deadline is managed dur-
ing the DATA and BDAT phase.

This feature is available in Postfix 3.7 and later. A weaker feature,
called smtpd_per_record_deadline, is available with Postfix 2.9-3.6.
With older Postfix releases, the behavior is as if this parameter is set
to "no".

This feature is available in Postfix 3.7 and later.

smtpd_policy_service_default_action (default: 451 4.3.5 Server configuration
problem)
The default action when an SMTPD policy service request fails. Specify
"DUNNO" to behave as if the failed SMTPD policy service request was not
sent, and to continue processing other access restrictions, if any.

Limitations:

• This parameter may specify any value that would be a valid SMTPD
policy server response (or access(5) map lookup result). An ac-
cess(5) map or policy server in this parameter value may need to
be declared in advance with a restriction_class setting.

• If the specified action invokes another check_policy_service re-
quest, that request will have the built-in default action.

This feature is available in Postfix 3.0 and later.

smtpd_policy_service_max_idle (default: 300s)
The time after which an idle SMTPD policy service connection is closed.

This feature is available in Postfix 2.1 and later.

smtpd_policy_service_max_ttl (default: 1000s)
The time after which an active SMTPD policy service connection is
closed.

This feature is available in Postfix 2.1 and later.

smtpd_policy_service_policy_context (default: empty)
Optional information that the Postfix SMTP server specifies in the "pol-
icy_context" attribute of a policy service request (originally, to share
the same service endpoint among multiple check_policy_service clients).

This feature is available in Postfix 3.1 and later.

smtpd_policy_service_request_limit (default: 0)
The maximal number of requests per SMTPD policy service connection, or
zero (no limit). Once a connection reaches this limit, the connection is
closed and the next request will be sent over a new connection. This is
a workaround to avoid error-recovery delays with policy servers that
cannot maintain a persistent connection.

This feature is available in Postfix 3.0 and later.

smtpd_policy_service_retry_delay (default: 1s)
The delay between attempts to resend a failed SMTPD policy service re-
quest. Specify a value greater than zero.

This feature is available in Postfix 3.0 and later.

smtpd_policy_service_timeout (default: 100s)
The time limit for connecting to, writing to, or receiving from a dele-
gated SMTPD policy server.

This feature is available in Postfix 2.1 and later.

smtpd_policy_service_try_limit (default: 2)
The maximal number of attempts to send an SMTPD policy service request
before giving up. Specify a value greater than zero.

This feature is available in Postfix 3.0 and later.

smtpd_proxy_ehlo (default: $myhostname)
How the Postfix SMTP server announces itself to the proxy filter. By
default, the Postfix hostname is used.

This feature is available in Postfix 2.1 and later.

smtpd_proxy_filter (default: empty)
The hostname and TCP port of the mail filtering proxy server. The proxy
receives all mail from the Postfix SMTP server, and is supposed to give
the result to another Postfix SMTP server process.

Specify "host:port" or "inet:host:port" for a TCP endpoint, or
"unix:pathname" for a UNIX-domain endpoint. The host can be specified as
an IP address or as a symbolic name; no MX lookups are done. When no
"host" or "host:" is specified, the local machine is assumed. Pathname
interpretation is relative to the Postfix queue directory.

This feature is available in Postfix 2.1 and later.

The "inet:" and "unix:" prefixes are available in Postfix 2.3 and later.

smtpd_proxy_options (default: empty)
List of options that control how the Postfix SMTP server communicates
with a before-queue content filter. Specify zero or more of the follow-
ing, separated by comma or whitespace.

speed_adjust
Do not connect to a before-queue content filter until an entire
message has been received. This reduces the number of simultane-
ous before-queue content filter processes.

NOTE 1: A filter must not selectively reject recipients of a multi-re-
cipient message. Rejecting all recipients is OK, as is accepting all
recipients.

NOTE 2: This feature increases the minimum amount of free queue space by
$message_size_limit. The extra space is needed to save the message to a
temporary file.

This feature is available in Postfix 2.7 and later.

smtpd_proxy_timeout (default: 100s)
The time limit for connecting to a proxy filter and for sending or re-
ceiving information. When a connection fails the client gets a generic
error message while more detailed information is logged to the maillog
file.

This feature is available in Postfix 2.1 and later.

smtpd_recipient_limit (default: 1000)
The maximal number of recipients that the Postfix SMTP server accepts
per message delivery request.

smtpd_recipient_overshoot_limit (default: 1000)
The number of recipients that a remote SMTP client can send in excess of
the limit specified with $smtpd_recipient_limit, before the Postfix SMTP
server increments the per-session error count for each excess recipient.

smtpd_recipient_restrictions (default: see postconf -d output)
Optional restrictions that the Postfix SMTP server applies in the con-
text of a client RCPT TO command, after smtpd_relay_restrictions. See
SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access restric-
tion lists" for a discussion of evaluation context and time.

With Postfix versions before 2.10, the rules for relay permission and
spam blocking were combined under smtpd_recipient_restrictions, result-
ing in error-prone configuration. As of Postfix 2.10, relay permission
rules are preferably implemented with smtpd_relay_restrictions, so that
a permissive spam blocking policy under smtpd_recipient_restrictions
will no longer result in a permissive mail relay policy.

For backwards compatibility, sites that migrate from Postfix versions
before 2.10 can set smtpd_relay_restrictions to the empty value, and use
smtpd_recipient_restrictions exactly as before.

IMPORTANT: Either the smtpd_relay_restrictions or the smtpd_recipi-
ent_restrictions parameter must specify at least one of the following
restrictions. Otherwise Postfix will refuse to receive mail:

reject, reject_unauth_destination

defer, defer_if_permit, defer_unauth_destination

The following restrictions are specific to the recipient address that is
received with the RCPT TO command.

check_recipient_access type:table
Search the specified access(5) database for the resolved RCPT TO
address, and execute the corresponding action.

check_recipient_a_access type:table
Search the specified access(5) database for the IP addresses for
the RCPT TO domain, and execute the corresponding action. Note:
a result of "OK" is not allowed for safety reasons. Instead, use
DUNNO in order to exclude specific hosts from denylists. This
feature is available in Postfix 3.0 and later.

check_recipient_mx_access type:table
Search the specified access(5) database for the MX hosts for the
RCPT TO domain, and execute the corresponding action. If no MX
record is found, look up A or AAAA records, just like the Postfix
SMTP client would. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific
hosts from denylists. This feature is available in Postfix 2.1
and later.

check_recipient_ns_access type:table
Search the specified access(5) database for the DNS servers for
the RCPT TO domain, and execute the corresponding action. Note:
a result of "OK" is not allowed for safety reasons. Instead, use
DUNNO in order to exclude specific hosts from denylists. This
feature is available in Postfix 2.1 and later.

permit_auth_destination
Permit the request when one of the following is true:

• Postfix is a mail forwarder: the resolved RCPT TO domain matches
$relay_domains or a subdomain thereof, and the address contains
no sender-specified routing (user@elsewhere@domain),

• Postfix is the final destination: the resolved RCPT TO domain
matches $mydestination, $inet_interfaces, $proxy_interfaces,
$virtual_alias_domains, or $virtual_mailbox_domains, and the ad-
dress contains no sender-specified routing (user@elsewhere@do-
main).

permit_mx_backup
Permit the request when the local mail system is a backup MX for
the RCPT TO domain, or when the domain is an authorized destina-
tion (see permit_auth_destination for definition).

• Safety: permit_mx_backup does not accept addresses that have
sender-specified routing information (example: user@elsewhere@do-
main).

• Safety: permit_mx_backup can be vulnerable to mis-use when access
is not restricted with permit_mx_backup_networks.

• Safety: as of Postfix version 2.3, permit_mx_backup no longer ac-
cepts the address when the local mail system is a primary MX for
the recipient domain. Exception: permit_mx_backup accepts the
address when it specifies an authorized destination (see per-
mit_auth_destination for definition).

• Limitation: mail may be rejected in case of a temporary DNS
lookup problem with Postfix prior to version 2.0.

reject_non_fqdn_recipient
Reject the request when the RCPT TO address specifies a domain
that is not in fully-qualified domain form, as required by the
RFC.
The non_fqdn_reject_code parameter specifies the response code
for rejected requests (default: 504).

reject_rhsbl_recipient rbl_domain=d.d.d.d
Reject the request when the RCPT TO domain is listed with the A
record "d.d.d.d" under rbl_domain (Postfix version 2.1 and later
only). Each "d" is a number, or a pattern inside "[]" that con-
tains one or more ";"-separated numbers or number..number ranges
(Postfix version 2.8 and later). If no "=d.d.d.d" is specified,
reject the request when the RCPT TO domain is listed with any A
record under rbl_domain.
The maps_rbl_reject_code parameter specifies the response code
for rejected requests (default: 554); the default_rbl_reply para-
meter specifies the default server reply; and the rbl_reply_maps
parameter specifies tables with server replies indexed by rbl_do-
main. This feature is available in Postfix version 2.0 and
later.

reject_unauth_destination
Reject the request unless one of the following is true:

• Postfix is a mail forwarder: the resolved RCPT TO domain matches
$relay_domains or a subdomain thereof, and contains no
sender-specified routing (user@elsewhere@domain),

• Postfix is the final destination: the resolved RCPT TO domain
matches $mydestination, $inet_interfaces, $proxy_interfaces,
$virtual_alias_domains, or $virtual_mailbox_domains, and contains
no sender-specified routing (user@elsewhere@domain).
The relay_domains_reject_code parameter specifies the response
code for rejected requests (default: 554).

defer_unauth_destination
Reject the same requests as reject_unauth_destination, with a
non-permanent error code. This feature is available in Postfix
2.10 and later.

reject_unknown_recipient_domain
Reject the request when Postfix is not final destination for the
recipient domain, and the RCPT TO domain has 1) no DNS MX and no
DNS A record or 2) a malformed MX record such as a record with a
zero-length MX hostname (Postfix version 2.3 and later).
The reply is specified with the unknown_address_reject_code para-
meter (default: 450), unknown_address_tempfail_action (default:
defer_if_permit), or 556 (nullmx, Postfix 3.0 and later). See the
respective parameter descriptions for details.

reject_unlisted_recipient (with Postfix version 2.0: check_recipi-
ent_maps)
Reject the request when the RCPT TO address is not listed in the
list of valid recipients for its domain class. See the smtpd_re-
ject_unlisted_recipient parameter description for details. This
feature is available in Postfix 2.1 and later.

reject_unverified_recipient
Reject the request when mail to the RCPT TO address is known to
bounce, or when the recipient address destination is not reach-
able. Address verification information is managed by the ver-
ify(8) server; see the ADDRESS_VERIFICATION_README file for de-
tails.
The unverified_recipient_reject_code parameter specifies the nu-
merical response code when an address is known to bounce (de-
fault: 450, change it to 550 when you are confident that it is
safe to do so).
The unverified_recipient_defer_code parameter specifies the nu-
merical response code when an address probe failed due to a tem-
porary problem (default: 450).
The unverified_recipient_tempfail_action parameter specifies the
action after address probe failure due to a temporary problem
(default: defer_if_permit).
This feature breaks for aliased addresses with "enable_origi-
nal_recipient = no" (Postfix <= 3.2).
This feature is available in Postfix 2.1 and later.

Other restrictions that are valid in this context:

• Generic restrictions that can be used in any SMTP command con-
text, described under smtpd_client_restrictions.

• SMTP command specific restrictions described under
smtpd_client_restrictions, smtpd_helo_restrictions and
smtpd_sender_restrictions.

Example:

# The Postfix before 2.10 default mail relay policy. Later Postfix
# versions implement this preferably with smtpd_relay_restrictions.
smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination

smtpd_reject_footer (default: empty)
Optional information that is appended after each Postfix SMTP server 4XX
or 5XX response.

The following example uses "\c" at the start of the template (supported
in Postfix 2.10 and later) to suppress the line break between the reply
text and the footer text. With earlier Postfix versions, the footer text
always begins on a new line, and the "\c" is output literally.

/etc/postfix/main.cf:
smtpd_reject_footer = \c. For assistance, call 800-555-0101.
Please provide the following information in your problem report:
time ($localtime), client ($client_address) and server
($server_name).

Server response:

550-5.5.1 <user@example> Recipient address rejected: User
unknown. For assistance, call 800-555-0101. Please provide the
following information in your problem report: time (Jan 4 15:42:00),
client (192.168.1.248) and server (mail1.example.com).

Note: the above text is meant to make it easier to find the Postfix log-
file records for a failed SMTP session. The text itself is not logged to
the Postfix SMTP server's maillog file.

Be sure to keep the text as short as possible. Long text may be trun-
cated before it is logged to the remote SMTP client's maillog file, or
before it is returned to the sender in a delivery status notification.

The template text is not subject to Postfix configuration parameter
$name expansion. Instead, this feature supports a limited number of
$name attributes in the footer text. These attributes are replaced with
their current value for the SMTP session.

Note: specify $$name in footer text that is looked up from regexp: or
pcre:-based smtpd_reject_footer_maps, otherwise the Postfix server will
not use the footer text and will log a warning instead.

client_address
The Client IP address that is logged in the maillog file.

client_port
The client TCP port that is logged in the maillog file.

localtime
The server local time (Mmm dd hh:mm:ss) that is logged in the
maillog file.

server_name
The server's myhostname value. This attribute is made available
for sites with multiple MTAs (perhaps behind a load-balancer),
where the server name can help the server support team to quickly
find the right log files.

Notes:

• NOT SUPPORTED are other attributes such as sender, recipient, or
main.cf parameters.

• For safety reasons, text that does not match $smtpd_expan-
sion_filter is censored.

This feature supports the two-character sequence \n as a request for a
line break in the footer text. Postfix automatically inserts after each
line break the three-digit SMTP reply code (and optional enhanced status
code) from the original Postfix reject message.

To work around mail software that mis-handles multi-line replies, spec-
ify the two-character sequence \c at the start of the template. This
suppresses the line break between the reply text and the footer text
(Postfix 2.10 and later).

This feature is available in Postfix 2.8 and later.

smtpd_reject_footer_maps (default: empty)
Lookup tables, indexed by the complete Postfix SMTP server 4xx or 5xx
response, with reject footer templates. See smtpd_reject_footer for de-
tails.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

This feature is available in Postfix 3.4 and later.

smtpd_reject_unlisted_recipient (default: yes)
Request that the Postfix SMTP server rejects mail for unknown recipient
addresses, even when no explicit reject_unlisted_recipient access re-
striction is specified. This prevents the Postfix queue from filling up
with undeliverable MAILER-DAEMON messages.

An address is considered "unknown" when 1) it does not match a vir-
tual(5) alias or canonical(5) mapping, and 2) the address is not valid
for its address class. For a definition of class-based address valida-
tion, see ADDRESS_CLASS_README.

This feature is available in Postfix 2.1 and later.

smtpd_reject_unlisted_sender (default: no)
Request that the Postfix SMTP server rejects mail from unknown sender
addresses, even when no explicit reject_unlisted_sender access restric-
tion is specified. This can slow down an explosion of forged mail from
worms or viruses.

This feature is available in Postfix 2.1 and later.

smtpd_relay_before_recipient_restrictions (default: see postconf -d output)
Evaluate smtpd_relay_restrictions before smtpd_recipient_restrictions.
Historically, smtpd_relay_restrictions was evaluated after smtpd_recipi-
ent_restrictions, contradicting documented behavior.

Background: the smtpd_relay_restrictions feature is primarily designed
to enforce a mail relaying policy, while smtpd_recipient_restrictions is
primarily designed to enforce spam blocking policy. Both are evaluated
while replying to the RCPT TO command, and both support the same fea-
tures.

This feature is available in Postfix 3.6 and later.

smtpd_relay_restrictions (default: permit_mynetworks, permit_sasl_authenti-
cated, defer_unauth_destination)
Access restrictions for mail relay control that the Postfix SMTP server
applies in the context of the RCPT TO command, before smtpd_recipi-
ent_restrictions. See SMTPD_ACCESS_README, section "Delayed evaluation
of SMTP access restriction lists" for a discussion of evaluation context
and time.

For backwards compatibility, sites that migrate from Postfix versions
before 2.10 can set smtpd_relay_restrictions to the empty value, and use
smtpd_recipient_restrictions exactly as before.

By default, the Postfix SMTP server accepts:

• Mail from clients whose IP address matches $mynetworks, or:

• Mail from clients who are SASL authenticated, or:

• Mail to remote destinations that match $relay_domains, except for
addresses that contain sender-specified routing (user@else-
where@domain), or:

• Mail to local destinations that match $inet_interfaces or
$proxy_interfaces, $mydestination, $virtual_alias_domains, or
$virtual_mailbox_domains.

IMPORTANT: Either the smtpd_relay_restrictions or the smtpd_recipi-
ent_restrictions parameter must specify at least one of the following
restrictions. Otherwise Postfix will refuse to receive mail:

reject, reject_unauth_destination

defer, defer_if_permit, defer_unauth_destination

Specify a list of restrictions, separated by commas and/or whitespace.
Continue long lines by starting the next line with whitespace. The same
restrictions are available as documented under smtpd_recipient_restric-
tions.

This feature is available in Postfix 2.10 and later.

smtpd_restriction_classes (default: empty)
User-defined aliases for groups of access restrictions. The aliases can
be specified in smtpd_recipient_restrictions etc., and on the right-hand
side of a Postfix access(5) table.

One major application is for implementing per-recipient UCE control.
See the RESTRICTION_CLASS_README document for other examples.

smtpd_sasl_application_name (default: smtpd)
The application name that the Postfix SMTP server uses for SASL server
initialization. This controls the name of the SASL configuration file.
The default value is smtpd, corresponding to a SASL configuration file
named smtpd.conf.

This feature is available in Postfix 2.1 and 2.2. With Postfix 2.3 it
was renamed to smtpd_sasl_path.

smtpd_sasl_auth_enable (default: no)
Enable SASL authentication in the Postfix SMTP server. By default, the
Postfix SMTP server does not use authentication.

If a remote SMTP client is authenticated, the permit_sasl_authenticated
access restriction can be used to permit relay access, like this:

# With Postfix 2.10 and later, the mail relay policy is
# preferably specified under smtpd_relay_restrictions.
smtpd_relay_restrictions =
permit_mynetworks, permit_sasl_authenticated, ...

# With Postfix before 2.10, the relay policy can be
# specified only under smtpd_recipient_restrictions.
smtpd_recipient_restrictions =
permit_mynetworks, permit_sasl_authenticated, ...

To reject all SMTP connections from unauthenticated clients, specify
"smtpd_delay_reject = yes" (which is the default) and use:

smtpd_client_restrictions = permit_sasl_authenticated, reject

See the SASL_README file for SASL configuration and operation details.

smtpd_sasl_authenticated_header (default: no)
Report the SASL authenticated user name in the smtpd(8) Received message
header.

This feature is available in Postfix 2.3 and later.

smtpd_sasl_exceptions_networks (default: empty)
What remote SMTP clients the Postfix SMTP server will not offer AUTH
support to.

Some clients (Netscape 4 at least) have a bug that causes them to re-
quire a login and password whenever AUTH is offered, whether it's neces-
sary or not. To work around this, specify, for example, $mynetworks to
prevent Postfix from offering AUTH to local clients.

Specify a list of network/netmask patterns, separated by commas and/or
whitespace. The mask specifies the number of bits in the network part of
a host address. You can also specify "/file/name" or "type:table" pat-
terns. A "/file/name" pattern is replaced by its contents; a "type:ta-
ble" lookup table is matched when a table entry matches a lookup string
(the lookup result is ignored). Continue long lines by starting the
next line with whitespace. Specify "!pattern" to exclude an address or
network block from the list. The form "!/file/name" is supported only
in Postfix version 2.4 and later.

Note: IP version 6 address information must be specified inside [] in
the smtpd_sasl_exceptions_networks value, and in files specified with
"/file/name". IP version 6 addresses contain the ":" character, and
would otherwise be confused with a "type:table" pattern.

Example:

smtpd_sasl_exceptions_networks = $mynetworks

This feature is available in Postfix 2.1 and later.

smtpd_sasl_local_domain (default: empty)
The name of the Postfix SMTP server's local SASL authentication realm.

By default, the local authentication realm name is the null string.

Examples:

smtpd_sasl_local_domain = $mydomain
smtpd_sasl_local_domain = $myhostname

smtpd_sasl_mechanism_filter (default: !external, static:rest)
If non-empty, a filter for the SASL mechanism names that the Postfix
SMTP server will announce in the EHLO response. By default, the Postfix
SMTP server will not announce the EXTERNAL mechanism, because Postfix
support for that is not implemented.

Specify mechanism names, "/file/name" patterns, or "type:table" lookup
tables, separated by comma or whitespace. The right-hand side result
from "type:table" lookups is ignored. Specify "!pattern" to exclude a
mechanism name from the list.

Examples:

smtpd_sasl_mechanism_filter = !external, !gssapi, static:rest
smtpd_sasl_mechanism_filter = login, plain
smtpd_sasl_mechanism_filter = /etc/postfix/smtpd_mechs

This feature is available in Postfix 3.6 and later.

smtpd_sasl_path (default: smtpd)
Implementation-specific information that the Postfix SMTP server passes
through to the SASL plug-in implementation that is selected with
smtpd_sasl_type. Typically this specifies the name of a configuration
file or rendezvous point.

This feature is available in Postfix 2.3 and later. In earlier releases
it was called smtpd_sasl_application_name.

smtpd_sasl_response_limit (default: 12288)
The maximum length of a SASL client's response to a server challenge.
When the client's "initial response" is longer than the normal limit for
SMTP commands, the client must omit its initial response, and wait for
an empty server challenge; it can then send what would have been its
"initial response" as a response to the empty server challenge. RFC4954
requires the server to accept client responses up to at least 12288
octets of base64-encoded text. The default value is therefore also the
minimum value accepted for this parameter.

This feature is available in Postfix 3.4 and later. Prior versions use
"line_length_limit", which may need to be raised to accommodate larger
client responses, as may be needed with GSSAPI authentication of Windows
AD users who are members of many groups.

smtpd_sasl_security_options (default: noanonymous)
Postfix SMTP server SASL security options; as of Postfix 2.3 the list of
available features depends on the SASL server implementation that is se-
lected with smtpd_sasl_type.

The following security features are defined for the cyrus server SASL
implementation:

Restrict what authentication mechanisms the Postfix SMTP server will of-
fer to the client. The list of available authentication mechanisms is
system dependent.

Specify zero or more of the following:

noplaintext
Disallow methods that use plaintext passwords.

noactive
Disallow methods subject to active (non-dictionary) attack.

nodictionary
Disallow methods subject to passive (dictionary) attack.

noanonymous
Disallow methods that allow anonymous authentication.

forward_secrecy
Only allow methods that support forward secrecy (Dovecot only).

mutual_auth
Only allow methods that provide mutual authentication (not avail-
able with Cyrus SASL version 1).

By default, the Postfix SMTP server accepts plaintext passwords but not
anonymous logins.

Warning: it appears that clients try authentication methods in the order
as advertised by the server (e.g., PLAIN ANONYMOUS CRAM-MD5) which means
that if you disable plaintext passwords, clients will log in anony-
mously, even when they should be able to use CRAM-MD5. So, if you dis-
able plaintext logins, disable anonymous logins too. Postfix treats
anonymous login as no authentication.

Example:

smtpd_sasl_security_options = noanonymous, noplaintext

smtpd_sasl_service (default: smtp)
The service name that is passed to the SASL plug-in that is selected
with smtpd_sasl_type and smtpd_sasl_path.

This feature is available in Postfix 2.11 and later. Prior versions be-
have as if "smtp" is specified.

smtpd_sasl_tls_security_options (default: $smtpd_sasl_security_options)
The SASL authentication security options that the Postfix SMTP server
uses for TLS encrypted SMTP sessions.

This feature is available in Postfix 2.2 and later.

smtpd_sasl_type (default: cyrus)
The SASL plug-in type that the Postfix SMTP server should use for au-
thentication. The available types are listed with the "postconf -a" com-
mand.

This feature is available in Postfix 2.3 and later.

smtpd_sender_login_maps (default: empty)
Optional lookup table with the SASL login names that own the envelope
sender (MAIL FROM) addresses.

Note: to enforce that the From: header address matches the envelope
sender (MAIL FROM) address, use an external filter such as a Milter,
for the submission or submissions (formerly called smtps ) services.
For example: https://github.com/magcks/milterfrom.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found. With lookups from indexed files such as DB or DBM, or from
networked tables such as NIS, LDAP or SQL, the following search opera-
tions are done with a sender address of user@domain:

1) user@domain
This table lookup is always done and has the highest precedence.

2) user
This table lookup is done only when the domain part of the sender
address matches $myorigin, $mydestination, $inet_interfaces or
$proxy_interfaces.

3) @domain
This table lookup is done last and has the lowest precedence.

In all cases the result of table lookup must be either "not found" or a
list of SASL login names separated by comma and/or whitespace.

smtpd_sender_restrictions (default: empty)
Optional restrictions that the Postfix SMTP server applies in the con-
text of a client MAIL FROM command. See SMTPD_ACCESS_README, section
"Delayed evaluation of SMTP access restriction lists" for a discussion
of evaluation context and time.

The default is to permit everything.

The following restrictions are specific to the sender address received
with the MAIL FROM command.

check_sender_access type:table
Search the specified access(5) database for the MAIL FROM ad-
dress, and execute the corresponding action.

check_sender_a_access type:table
Search the specified access(5) database for the IP addresses for
the MAIL FROM domain, and execute the corresponding action.
Note: a result of "OK" is not allowed for safety reasons. In-
stead, use DUNNO in order to exclude specific hosts from
denylists. This feature is available in Postfix 3.0 and later.

check_sender_mx_access type:table
Search the specified access(5) database for the MX hosts for the
MAIL FROM domain, and execute the corresponding action. If no MX
record is found, look up A or AAAA records, just like the Postfix
SMTP client would. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific
hosts from denylists. This feature is available in Postfix 2.1
and later.

check_sender_ns_access type:table
Search the specified access(5) database for the DNS servers for
the MAIL FROM domain, and execute the corresponding action.
Note: a result of "OK" is not allowed for safety reasons. In-
stead, use DUNNO in order to exclude specific hosts from
denylists. This feature is available in Postfix 2.1 and later.

reject_authenticated_sender_login_mismatch
Reject the request when the client is authenticated with SASL,
but either the MAIL FROM address is not listed in
$smtpd_sender_login_maps, or the SASL login name is not an owner
for that address.
This prevents an authenticated client from using a MAIL FROM ad-
dress that they do not explicitly own.
Note: to enforce that the From: header address matches the enve-
lope sender (MAIL FROM) address, use an external filter such as a
Milter, for the submission or submissions (formerly called smtps)
services. For example: https://github.com/magcks/milterfrom.
This feature is available in Postfix version 2.1 and later.

reject_known_sender_login_mismatch
When the client is authenticated with SASL, reject the request
when the MAIL FROM address is listed in $smtpd_sender_login_maps,
but the SASL login name is not an owner for that address.
When the client is not authenticated with SASL, reject the re-
quest when SASL is enabled, and the MAIL FROM address is listed
in $smtpd_sender_login_maps.
This protects any MAIL FROM address that is listed in
$smtpd_sender_login_maps, while still allowing a client to use
any unlisted MAIL FROM address.
Note: to enforce that the From: header address matches the enve-
lope sender (MAIL FROM) address, use an external filter such as a
Milter, for the submission or submissions (formerly called smtps)
services. For example: https://github.com/magcks/milterfrom.
This feature is available in Postfix version 2.11 and later.

reject_non_fqdn_sender
Reject the request when the MAIL FROM address specifies a domain
that is not in fully-qualified domain form as required by the
RFC.
The non_fqdn_reject_code parameter specifies the response code
for rejected requests (default: 504).

reject_rhsbl_sender rbl_domain=d.d.d.d
Reject the request when the MAIL FROM domain is listed with the A
record "d.d.d.d" under rbl_domain (Postfix version 2.1 and later
only). Each "d" is a number, or a pattern inside "[]" that con-
tains one or more ";"-separated numbers or number..number ranges
(Postfix version 2.8 and later). If no "=d.d.d.d" is specified,
reject the request when the MAIL FROM domain is listed with any A
record under rbl_domain.
The maps_rbl_reject_code parameter specifies the response code
for rejected requests (default: 554); the default_rbl_reply pa-
rameter specifies the default server reply; and the rbl_re-
ply_maps parameter specifies tables with server replies indexed
by rbl_domain. This feature is available in Postfix 2.0 and
later.

reject_sender_login_mismatch
As of Postfix 2.1, this is an alias for "reject_authenti-
cated_sender_login_mismatch, reject_unauthenticated_sender_lo-
gin_mismatch".

reject_unauthenticated_sender_login_mismatch
Reject the request when SASL is enabled, the MAIL FROM address is
listed in $smtpd_sender_login_maps, but the client is not authen-
ticated with SASL.
With SASL enabled, this prevents an unauthenticated client from
using any MAIL FROM address that is listed in $smtpd_sender_lo-
gin_maps.
Note: to enforce that the From: header address matches the enve-
lope sender (MAIL FROM) address, use an external filter such as a
Milter, for the submission or submissions (formerly called smtps)
services. For example: https://github.com/magcks/milterfrom.
This feature is available in Postfix version 2.1 and later.

reject_unknown_sender_domain
Reject the request when Postfix is not the final destination for
the sender address, and the MAIL FROM domain has 1) no DNS MX and
no DNS A record, or 2) a malformed MX record such as a record
with a zero-length MX hostname (Postfix version 2.3 and later).
The reply is specified with the unknown_address_reject_code para-
meter (default: 450), unknown_address_tempfail_action (default:
defer_if_permit), or 550 (nullmx, Postfix 3.0 and later). See the
respective parameter descriptions for details.

reject_unlisted_sender
Reject the request when the MAIL FROM address is not listed in
the list of valid recipients for its domain class. See the
smtpd_reject_unlisted_sender parameter description for details.
This feature is available in Postfix 2.1 and later.

reject_unverified_sender
Reject the request when mail to the MAIL FROM address is known to
bounce, or when the sender address destination is not reachable.
Address verification information is managed by the verify(8)
server; see the ADDRESS_VERIFICATION_README file for details.
The unverified_sender_reject_code parameter specifies the numeri-
cal response code when an address is known to bounce (default:
450, change into 550 when you are confident that it is safe to do
so).
The unverified_sender_defer_code specifies the numerical response
code when an address probe failed due to a temporary problem (de-
fault: 450).
The unverified_sender_tempfail_action parameter specifies the ac-
tion after address probe failure due to a temporary problem (de-
fault: defer_if_permit).
This feature breaks for aliased addresses with "enable_origi-
nal_recipient = no" (Postfix <= 3.2).
This feature is available in Postfix 2.1 and later.

Other restrictions that are valid in this context:

• Generic restrictions that can be used in any SMTP command con-
text, described under smtpd_client_restrictions.

• SMTP command specific restrictions described under
smtpd_client_restrictions and smtpd_helo_restrictions.

• SMTP command specific restrictions described under smtpd_recipi-
ent_restrictions. When recipient restrictions are listed under
smtpd_sender_restrictions, they have effect only with "smtpd_de-
lay_reject = yes", so that $smtpd_sender_restrictions is evalu-
ated at the time of the RCPT TO command.

Examples:

smtpd_sender_restrictions = reject_unknown_sender_domain
smtpd_sender_restrictions = reject_unknown_sender_domain,
check_sender_access hash:/etc/postfix/access

smtpd_service_name (default: smtpd)
The internal service that postscreen(8) hands off allowed connections
to. In a future version there may be different classes of SMTP service.

This feature is available in Postfix 2.8.

smtpd_soft_error_limit (default: 10)
The number of errors a remote SMTP client is allowed to make without de-
livering mail before the Postfix SMTP server slows down all its re-
sponses.

• With Postfix version 2.1 and later, when the error count is >
$smtpd_soft_error_limit, the Postfix SMTP server delays all re-
sponses by $smtpd_error_sleep_time.

• With Postfix versions 2.0 and earlier, when the error count is >
$smtpd_soft_error_limit, the Postfix SMTP server delays all re-
sponses by the larger of (number of errors) seconds or $smtpd_er-
ror_sleep_time.

• With Postfix versions 2.0 and earlier, when the error count is <=
$smtpd_soft_error_limit, the Postfix SMTP server delays 4XX and
5XX responses by $smtpd_error_sleep_time.

smtpd_starttls_timeout (default: see postconf -d output)
The time limit for Postfix SMTP server write and read operations during
TLS startup and shutdown handshake procedures. The current default value
is stress-dependent. Before Postfix version 2.8, it was fixed at 300s.

This feature is available in Postfix 2.2 and later.

smtpd_timeout (default: normal: 300s, overload: 10s)
When the Postfix SMTP server wants to send an SMTP server response, how
long the Postfix SMTP server will wait for an underlying network write
operation to complete; and when the Postfix SMTP server Postfix wants to
receive an SMTP client request, how long the Postfix SMTP server will
wait for an underlying network read operation to complete. See the
smtpd_per_request_deadline for how this time limit may be enforced (with
Postfix 2.9-3.6 see smtpd_per_record_deadline).

Normally the default limit is 300s, but it changes under overload to
just 10s. With Postfix 2.5 and earlier, the SMTP server always uses a
time limit of 300s by default.

Note: if you set SMTP time limits to very large values you may have to
update the global ipc_timeout parameter.

smtpd_tls_CAfile (default: empty)
A file containing (PEM format) CA certificates of root CAs trusted to
sign either remote SMTP client certificates or intermediate CA certifi-
cates. These are loaded into memory before the smtpd(8) server enters
the chroot jail. If the number of trusted roots is large, consider using
smtpd_tls_CApath instead, but note that the latter directory must be
present in the chroot jail if the smtpd(8) server is chrooted. This file
may also be used to augment the server certificate trust chain, but it
is best to include all the required certificates directly in the server
certificate file.

Specify "smtpd_tls_CAfile = /path/to/system_CA_file" to use ONLY the
system-supplied default Certification Authority certificates.

Specify "tls_append_default_CA = no" to prevent Postfix from appending
the system-supplied default CAs and trusting third-party certificates.

By default (see smtpd_tls_ask_ccert), client certificates are not re-
quested, and smtpd_tls_CAfile should remain empty. If you do make use of
client certificates, the distinguished names (DNs) of the Certification
Authorities listed in smtpd_tls_CAfile are sent to the remote SMTP
client in the client certificate request message. MUAs with multiple
client certificates may use the list of preferred Certification Authori-
ties to select the correct client certificate. You may want to put your
"preferred" CA or CAs in this file, and install other trusted CAs in
$smtpd_tls_CApath.

Example:

smtpd_tls_CAfile = /etc/postfix/CAcert.pem

This feature is available in Postfix 2.2 and later.

smtpd_tls_CApath (default: empty)
A directory containing (PEM format) CA certificates of root CAs trusted
to sign either remote SMTP client certificates or intermediate CA cer-
tificates. Do not forget to create the necessary "hash" links with, for
example, "$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs". To use
smtpd_tls_CApath in chroot mode, this directory (or a copy) must be in-
side the chroot jail.

Specify "smtpd_tls_CApath = /path/to/system_CA_directory" to use ONLY
the system-supplied default Certification Authority certificates.

Specify "tls_append_default_CA = no" to prevent Postfix from appending
the system-supplied default CAs and trusting third-party certificates.

By default (see smtpd_tls_ask_ccert), client certificates are not re-
quested, and smtpd_tls_CApath should remain empty. In contrast to
smtpd_tls_CAfile, DNs of Certification Authorities installed in
$smtpd_tls_CApath are not included in the client certificate request
message. MUAs with multiple client certificates may use the list of pre-
ferred Certification Authorities to select the correct client certifi-
cate. You may want to put your "preferred" CA or CAs in
$smtpd_tls_CAfile, and install the remaining trusted CAs in
$smtpd_tls_CApath.

Example:

smtpd_tls_CApath = /etc/postfix/certs

This feature is available in Postfix 2.2 and later.

smtpd_tls_always_issue_session_ids (default: yes)
Force the Postfix SMTP server to issue a TLS session id, even when TLS
session caching is turned off (smtpd_tls_session_cache_database is
empty). This behavior is compatible with Postfix < 2.3.

With Postfix 2.3 and later the Postfix SMTP server can disable session
id generation when TLS session caching is turned off. This keeps remote
SMTP clients from caching sessions that almost certainly cannot be
re-used.

By default, the Postfix SMTP server always generates TLS session ids.
This works around a known defect in mail client applications such as MS
Outlook, and may also prevent interoperability issues with other MTAs.

Example:

smtpd_tls_always_issue_session_ids = no

This feature is available in Postfix 2.3 and later.

smtpd_tls_ask_ccert (default: no)
Ask a remote SMTP client for a client certificate. This information is
needed for certificate based mail relaying with, for example, the per-
mit_tls_clientcerts feature.

Some clients such as Netscape will either complain if no certificate is
available (for the list of CAs in $smtpd_tls_CAfile) or will offer mul-
tiple client certificates to choose from. This may be annoying, so this
option is "off" by default.

This feature is available in Postfix 2.2 and later.

smtpd_tls_auth_only (default: no)
When TLS encryption is optional in the Postfix SMTP server, do not an-
nounce or accept SASL authentication over unencrypted connections.

This feature is available in Postfix 2.2 and later.

smtpd_tls_ccert_verifydepth (default: 9)
The verification depth for remote SMTP client certificates. A depth of 1
is sufficient if the issuing CA is listed in a local CA file.

This feature is available in Postfix 2.2 and later.

smtpd_tls_cert_file (default: empty)
File with the Postfix SMTP server RSA certificate in PEM format. This
file may also contain the Postfix SMTP server private RSA key. With
Postfix >= 3.4 the preferred way to configure server keys and certifi-
cates is via the "smtpd_tls_chain_files" parameter.

Public Internet MX hosts without certificates signed by a "reputable" CA
must generate, and be prepared to present to most clients, a self-signed
or private-CA signed certificate. The client will not be able to authen-
ticate the server, but unless it is running Postfix 2.3 or similar soft-
ware, it will still insist on a server certificate.

For servers that are not public Internet MX hosts, Postfix supports con-
figurations with no certificates. This entails the use of just the
anonymous TLS ciphers, which are not supported by typical SMTP clients.
Since some clients may not fall back to plain text after a TLS handshake
failure, a certificate-less Postfix SMTP server will be unable to re-
ceive email from some TLS-enabled clients. To avoid accidental configu-
rations with no certificates, Postfix enables certificate-less operation
only when the administrator explicitly sets "smtpd_tls_cert_file =
none". This ensures that new Postfix SMTP server configurations will not
accidentally enable TLS without certificates.

Note that server certificates are not optional in TLS 1.3. To run with-
out certificates you'd have to disable the TLS 1.3 protocol by including
'!TLSv1.3' in "smtpd_tls_protocols" and perhaps also "smtpd_tls_manda-
tory_protocols". It is simpler instead to just configure a certificate
chain. Certificate-less operation is not recommended.

Both RSA and DSA certificates are supported. When both types are
present, the cipher used determines which certificate will be presented
to the client. For Netscape and OpenSSL clients without special cipher
choices the RSA certificate is preferred.

To enable a remote SMTP client to verify the Postfix SMTP server cer-
tificate, the issuing CA certificates must be made available to the
client. You should include the required certificates in the server cer-
tificate file, the server certificate first, then the issuing CA(s)
(bottom-up order).

Example: the certificate for "server.example.com" was issued by "inter-
mediate CA" which itself has a certificate of "root CA". Create the
server.pem file with "cat server_cert.pem intermediate_CA.pem
root_CA.pem > server.pem".

If you also want to verify client certificates issued by these CAs, you
can add the CA certificates to the smtpd_tls_CAfile, in which case it is
not necessary to have them in the smtpd_tls_cert_file,
smtpd_tls_dcert_file (obsolete) or smtpd_tls_eccert_file.

A certificate supplied here must be usable as an SSL server certificate
and hence pass the "openssl verify -purpose sslserver ..." test.

Example:

smtpd_tls_cert_file = /etc/postfix/server.pem

This feature is available in Postfix 2.2 and later.

smtpd_tls_chain_files (default: empty)
List of one or more PEM files, each holding one or more private keys di-
rectly followed by a corresponding certificate chain. The file names
are separated by commas and/or whitespace. This parameter obsoletes the
legacy algorithm-specific key and certificate file settings. When this
parameter is non-empty, the legacy parameters are ignored, and a warning
is logged if any are also non-empty.

RSA is still the most widely supported algorithm. Presently (late
2018), ECDSA support is common, but not yet universal, and Ed25519 and
Ed448 support is mostly absent. Therefore, an RSA key should generally
be configured, along with any additional keys for the other algorithms
when desired.

Example (separate files for each key and corresponding certificate
chain):

/etc/postfix/main.cf:
smtpd_tls_chain_files =
${config_directory}/ed25519.pem,
${config_directory}/ed448.pem,
${config_directory}/rsa.pem

Example (all keys and certificates in a single file):

/etc/postfix/main.cf:
smtpd_tls_chain_files = ${config_directory}/chains.pem

This feature is available in Postfix 3.4 and later.

smtpd_tls_cipherlist (default: empty)
Obsolete Postfix < 2.3 control for the Postfix SMTP server TLS cipher
list. It is easy to create interoperability problems by choosing a
non-default cipher list. Do not use a non-default TLS cipherlist for MX
hosts on the public Internet. Clients that begin the TLS handshake, but
are unable to agree on a common cipher, may not be able to send any
email to the SMTP server. Using a restricted cipher list may be more ap-
propriate for a dedicated MSA or an internal mailhub, where one can ex-
ert some control over the TLS software and settings of the connecting
clients.

Note: do not use "" quotes around the parameter value.

This feature is available with Postfix version 2.2. It is not used with
Postfix 2.3 and later; use smtpd_tls_mandatory_ciphers instead.

smtpd_tls_ciphers (default: medium)
The minimum TLS cipher grade that the Postfix SMTP server will use with
opportunistic TLS encryption. Cipher types listed in smtpd_tls_ex-
clude_ciphers are excluded from the base definition of the selected ci-
pher grade. The default value is "medium" for Postfix releases after
the middle of 2015, "export" for older releases.

When TLS is mandatory the cipher grade is chosen via the
smtpd_tls_mandatory_ciphers configuration parameter, see there for syn-
tax details.

This feature is available in Postfix 2.6 and later. With earlier Postfix
releases only the smtpd_tls_mandatory_ciphers parameter is implemented,
and opportunistic TLS always uses "export" or better (i.e. all) ciphers.

smtpd_tls_dcert_file (default: empty)
File with the Postfix SMTP server DSA certificate in PEM format. This
file may also contain the Postfix SMTP server private DSA key. The DSA
algorithm is obsolete and should not be used.

See the discussion under smtpd_tls_cert_file for more details.

Example:

smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem

This feature is available in Postfix 2.2 and later.

smtpd_tls_dh1024_param_file (default: empty)
File with DH parameters that the Postfix SMTP server should use with
non-export EDH ciphers.

With Postfix >= 3.7, built with OpenSSL version is 3.0.0 or later, if
the parameter value is either empty or "auto", then the DH parameter se-
lection is delegated to the OpenSSL library, which selects appropriate
parameters based on the TLS handshake. This choice is likely to be the
most interoperable with SMTP clients using various TLS libraries, and
custom local parameters are no longer recommended when using Postfix >=
3.7 built against OpenSSL 3.0.0.

The best-practice choice of parameters uses a 2048-bit prime. This is
fine, despite the historical "1024" in the parameter name. Do not be
tempted to use much larger values, performance degrades quickly, and you
may also cease to interoperate with some mainstream SMTP clients. As of
Postfix 3.1, the compiled-in default prime is 2048-bits, and it is not
strictly necessary, though perhaps somewhat beneficial to generate cus-
tom DH parameters.

Instead of using the exact same parameter sets as distributed with other
TLS packages, it is more secure to generate your own set of parameters
with something like the following commands:

openssl dhparam -out /etc/postfix/dh2048.pem 2048
openssl dhparam -out /etc/postfix/dh1024.pem 1024
# As of Postfix 3.6, export-grade 512-bit DH parameters are no longer
# supported or needed.
openssl dhparam -out /etc/postfix/dh512.pem 512

It is safe to share the same DH parameters between multiple Postfix in-
stances. If you prefer, you can generate separate parameters for each
instance.

If you want to take maximal advantage of ciphers that offer forward se-
crecy see the Getting started section of FORWARD_SECRECY_README. The
full document conveniently presents all information about Postfix "per-
fect" forward secrecy support in one place: what forward secrecy is, how
to tweak settings, and what you can expect to see when Postfix uses ci-
phers with forward secrecy.

Example:

smtpd_tls_dh1024_param_file = /etc/postfix/dh2048.pem

This feature is available in Postfix 2.2 and later.

smtpd_tls_dh512_param_file (default: empty)
File with DH parameters that the Postfix SMTP server should use with ex-
port-grade EDH ciphers. The default SMTP server cipher grade is
"medium" with Postfix releases after the middle of 2015, and as a result
export-grade cipher suites are by default not used.

With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is no
longer supported, and this parameter is silently ignored.

See also the discussion under the smtpd_tls_dh1024_param_file configura-
tion parameter.

Example:

smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem

This feature is available in Postfix 2.2 and later, but is ignored in
Postfix 3.6 and later.

smtpd_tls_dkey_file (default: $smtpd_tls_dcert_file)
File with the Postfix SMTP server DSA private key in PEM format. This
file may be combined with the Postfix SMTP server DSA certificate file
specified with $smtpd_tls_dcert_file. The DSA algorithm is obsolete and
should not be used.

This feature is available in Postfix 2.2 and later.

smtpd_tls_eccert_file (default: empty)
File with the Postfix SMTP server ECDSA certificate in PEM format. This
file may also contain the Postfix SMTP server private ECDSA key. With
Postfix >= 3.4 the preferred way to configure server keys and certifi-
cates is via the "smtpd_tls_chain_files" parameter.

See the discussion under smtpd_tls_cert_file for more details.

Example:

smtpd_tls_eccert_file = /etc/postfix/ecdsa-scert.pem

This feature is available in Postfix 2.6 and later, when Postfix is com-
piled and linked with OpenSSL 1.0.0 or later.

smtpd_tls_eckey_file (default: $smtpd_tls_eccert_file)
File with the Postfix SMTP server ECDSA private key in PEM format. This
file may be combined with the Postfix SMTP server ECDSA certificate file
specified with $smtpd_tls_eccert_file. With Postfix >= 3.4 the pre-
ferred way to configure server keys and certificates is via the
"smtpd_tls_chain_files" parameter.

This feature is available in Postfix 2.6 and later, when Postfix is com-
piled and linked with OpenSSL 1.0.0 or later.

smtpd_tls_eecdh_grade (default: see postconf -d output)
The Postfix SMTP server security grade for ephemeral elliptic-curve
Diffie-Hellman (EECDH) key exchange. As of Postfix 3.6, the value of
this parameter is always ignored, and Postfix behaves as though the auto
value (described below) was chosen.

This feature is not used as of Postfix 3.6. Do not specify.

The available choices are:

auto Use the most preferred curve that is supported by both the client
and the server. This setting requires Postfix >= 3.2 compiled
and linked with OpenSSL >= 1.0.2. This is the default setting
under the above conditions (and the only setting used with Post-
fix >= 3.6).

none Don't use EECDH. Ciphers based on EECDH key exchange will be dis-
abled. This is the default in Postfix versions 2.6 and 2.7.

strong Use EECDH with approximately 128 bits of security at a reasonable
computational cost. This is the default in Postfix versions
2.8-3.5.

ultra Use EECDH with approximately 192 bits of security at computa-
tional cost that is approximately twice as high as 128 bit
strength ECC.

This feature is available in Postfix 2.6 and later, when it is compiled
and linked with OpenSSL 1.0.0 or later on platforms where EC algorithms
have not been disabled by the vendor.

smtpd_tls_enable_rpk (default: no)
Request that remote SMTP clients send an RFC7250 raw public key instead
of an X.509 certificate, when asking for or requiring client authentica-
tion. This feature is ignored when there is no raw public key support in
the local TLS implementation.

The Postfix SMTP server will log a warning when "smtpd_tls_enable_rpk =
yes", but the remote SMTP client sends a certificate, the certificate's
public key fingerprint does not match a check_ccert_access table, while
the certificate fingerprint does match a check_ccert_access table. The
remote SMTP client would lose access when it starts sending a raw public
key instead of a certificate, after its TLS implementation is updated
with raw public key support.

The Postfix SMTP server always sends a raw public key instead of a cer-
tificate, if solicited by the remote SMTP client and the local TLS im-
plementation supports raw public keys. If the client sends a server name
indication with an SNI TLS extension, and tls_server_sni_maps is config-
ured, the server will extract a raw public key from the indicated cer-
tificate.

Sample commands to compute certificate and public key SHA256 digests:

# SHA256 digest of the first certificate in "cert.pem"
$ openssl x509 -in cert.pem -outform DER | openssl dgst -sha256 -c

# SHA256 digest of the SPKI of the first certificate in "cert.pem"
$ openssl x509 -in cert.pem -pubkey -noout |
openssl pkey -pubin -outform DER | openssl dgst -sha256 -c

# SHA256 digest of the SPKI of the first private key in "pkey.pem"
$ openssl pkey -in pkey.pem -pubout -outform DER |
openssl dgst -sha256 -c

This feature is available in Postfix 3.9 and later.

smtpd_tls_exclude_ciphers (default: empty)
List of ciphers or cipher types to exclude from the SMTP server cipher
list at all TLS security levels. Excluding valid ciphers can create in-
teroperability problems. DO NOT exclude ciphers unless it is essential
to do so. This is not an OpenSSL cipherlist; it is a simple list sepa-
rated by whitespace and/or commas. The elements are a single cipher, or
one or more "+" separated cipher properties, in which case only ciphers
matching all the properties are excluded.

Examples (some of these will cause problems):

smtpd_tls_exclude_ciphers = aNULL
smtpd_tls_exclude_ciphers = MD5, DES
smtpd_tls_exclude_ciphers = DES+MD5
smtpd_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
smtpd_tls_exclude_ciphers = kEDH+aRSA

This feature is available in Postfix 2.3 and later.

smtpd_tls_fingerprint_digest (default: see postconf -d output)
The message digest algorithm to construct remote SMTP client-certificate
fingerprints or public key fingerprints (Postfix 2.9 and later) for
check_ccert_access and permit_tls_clientcerts.

The default algorithm is sha256 with Postfix >= 3.6 and the compatibil-
ity_level set to 3.6 or higher. With Postfix <= 3.5, the default algo-
rithm is md5.

To find the fingerprint of a specific certificate file, with a specific
digest algorithm, run:

$ openssl x509 -noout -fingerprint -digest -in certfile.pem

The text to the right of "=" sign is the desired fingerprint. For exam-
ple:

$ openssl x509 -noout -fingerprint -sha256 -in cert.pem
SHA256 Fingerprint=D4:6A:AB:19:24:...:A6:CB:66:82:C0:8E:9B:EE:29:A8:1A

Example:

$ openssl x509 -in cert.pem -noout -pubkey |
openssl pkey -pubin -outform DER |
openssl dgst -sha256 -c
(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:8b:fc:09:1a:61:98:b5:bc:7c:60:58

The Postfix SMTP server and client log the peer (leaf) certificate fin-
gerprint and public key fingerprint when the TLS loglevel is 2 or
higher.

Example: client-certificate access table, with sha256 fingerprints:

/etc/postfix/main.cf:
smtpd_tls_fingerprint_digest = sha256
smtpd_client_restrictions =
check_ccert_access hash:/etc/postfix/access,
reject
/etc/postfix/access:
# Action folded to next line...
AF:88:7C:AD:51:95:6F:36:96:...:01:FB:2E:48:CD:AB:49:25:A2:3B
OK
85:16:78:FD:73:6E:CE:70:E0:...:5F:0D:3C:C8:6D:C4:2C:24:59:E1
permit_auth_destination

This feature is available in Postfix 2.5 and later.

smtpd_tls_key_file (default: $smtpd_tls_cert_file)
File with the Postfix SMTP server RSA private key in PEM format. This
file may be combined with the Postfix SMTP server RSA certificate file
specified with $smtpd_tls_cert_file. With Postfix >= 3.4 the preferred
way to configure server keys and certificates is via the
"smtpd_tls_chain_files" parameter.

smtpd_tls_loglevel (default: 0)
Enable additional Postfix SMTP server logging of TLS activity. Each
logging level also includes the information that is logged at a lower
logging level.

0 Disable logging of TLS activity.

1 Log only a summary message on TLS handshake completion - no
logging of client certificate trust-chain verification errors if
client certificate verification is not required. With Postfix
2.8 and earlier, log the summary message, peer certificate sum-
mary information and unconditionally log trust-chain verification
errors.

2 Also enable verbose logging in the Postfix TLS library, log
session cache operations, and enable OpenSSL logging of the
progress of the SSL handshake.

3 Also log hexadecimal and ASCII dump of TLS negotiation process.

4 Also log hexadecimal and ASCII dump of complete transmission
after STARTTLS.

Do not use "smtpd_tls_loglevel = 2" or higher except in case of prob-
lems. Use of loglevel 4 is strongly discouraged.

This feature is available in Postfix 2.2 and later.

smtpd_tls_mandatory_ciphers (default: medium)
The minimum TLS cipher grade that the Postfix SMTP server will use with
mandatory TLS encryption. The default grade ("medium") is sufficiently
strong that any benefit from globally restricting TLS sessions to a more
stringent grade is likely negligible, especially given the fact that
many implementations still do not offer any stronger ("high" grade) ci-
phers, while those that do, will always use "high" grade ciphers. So in-
sisting on "high" grade ciphers is generally counter-productive. Allow-
ing "export" or "low" ciphers is typically not a good idea, as systems
limited to just these are limited to obsolete browsers. No known SMTP
clients fail to support at least one "medium" or "high" grade cipher.

The following cipher grades are supported:

high Enable only "HIGH" grade OpenSSL ciphers. The underlying ci-
pherlist is specified via the tls_high_cipherlist configuration
parameter, which you are strongly encouraged to not change.

medium Enable "MEDIUM" grade or stronger OpenSSL ciphers. These use
128-bit or longer symmetric bulk-encryption keys. This is the de-
fault minimum strength for mandatory TLS encryption. The underly-
ing cipherlist is specified via the tls_medium_cipherlist config-
uration parameter, which you are strongly encouraged not to
change.

null Enable only the "NULL" OpenSSL ciphers, these provide authentica-
tion without encryption. This setting is only appropriate in the
rare case that all clients are prepared to use NULL ciphers (not
normally enabled in TLS clients). The underlying cipherlist is
specified via the tls_null_cipherlist configuration parameter,
which you are strongly encouraged not to change.

Cipher types listed in smtpd_tls_mandatory_exclude_ciphers or
smtpd_tls_exclude_ciphers are excluded from the base definition of the
selected cipher grade. See smtpd_tls_ciphers for cipher controls that
apply to opportunistic TLS.

The underlying cipherlists for grades other than "null" include anony-
mous ciphers, but these are automatically filtered out if the server is
configured to ask for remote SMTP client certificates. You are very un-
likely to need to take any steps to exclude anonymous ciphers, they are
excluded automatically as required. If you must exclude anonymous ci-
phers even when Postfix does not need or use peer certificates, set
"smtpd_tls_exclude_ciphers = aNULL". To exclude anonymous ciphers only
when TLS is enforced, set "smtpd_tls_mandatory_exclude_ciphers = aNULL".

This feature is available in Postfix 2.3 and later.

smtpd_tls_mandatory_exclude_ciphers (default: empty)
Additional list of ciphers or cipher types to exclude from the Postfix
SMTP server cipher list at mandatory TLS security levels. This list
works in addition to the exclusions listed with smtpd_tls_exclude_ci-
phers (see there for syntax details).

This feature is available in Postfix 2.3 and later.

smtpd_tls_mandatory_protocols (default: see postconf -d output)
TLS protocols accepted by the Postfix SMTP server with mandatory TLS en-
cryption. If the list is empty, the server supports all available TLS
protocol versions. A non-empty value is a list of protocol names to in-
clude or exclude, separated by whitespace, commas or colons.

As of Postfix 3.6, the preferred way to limit the range of acceptable
protocols is to set the lowest acceptable TLS protocol version and/or
the highest acceptable TLS protocol version. To set the lower bound in-
clude an element of the form: ">=version" where version is a either one
of the TLS protocol names listed above, or a hexadecimal number corre-
sponding to the desired TLS protocol version (0301 for TLS 1.0, 0302 for
TLS 1.1, etc.). For the upper bound, use "<=version". There must be no
whitespace between the ">=" or "<=" symbols and the protocol name or
number.

Hexadecimal example (Postfix >= 3.6):

# Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtpd_tls_mandatory_protocols = >=TLSv1.2, <=0305
# Allow only TLS 1.2 and up:
smtpd_tls_mandatory_protocols = >=0x0303

With Postfix < 3.6 there is no support for a minimum or maximum version,
and the protocol range is configured via protocol exclusions. To re-
quire at least TLS 1.0, set "smtpd_tls_mandatory_protocols = !SSLv2,
!SSLv3". Listing the protocols to include, rather than protocols to ex-
clude, is supported, but not recommended. The exclusion form more accu-
rately matches the underlying OpenSSL interface.

Support for "TLSv1.3" was introduced in OpenSSL 1.1.1. Disabling this
protocol via "!TLSv1.3" is supported since Postfix 3.4 (or patch re-
leases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).

Example:

# Preferred syntax with Postfix >= 3.6:
smtpd_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
# Legacy syntax:
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

This feature is available in Postfix 2.3 and later.

smtpd_tls_protocols (default: see 'postconf -d' output)
TLS protocols accepted by the Postfix SMTP server with opportunistic TLS
encryption. If the list is empty, the server supports all available TLS
protocol versions. A non-empty value is a list of protocol names to in-
clude or exclude, separated by whitespace, commas or colons.

As of Postfix 3.6, the preferred way to limit the range of acceptable
protocols is to set the lowest acceptable TLS protocol version and/or
the highest acceptable TLS protocol version. To set the lower bound in-
clude an element of the form: ">=version" where version is a either one
of the TLS protocol names listed above, or a hexadecimal number corre-
sponding to the desired TLS protocol version (0301 for TLS 1.0, 0302 for
TLS 1.1, etc.). For the upper bound, use "<=version". There must be no
whitespace between the ">=" or "<=" symbols and the protocol name or
number.

Hexadecimal example (Postfix >= 3.6):

# Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtpd_tls_protocols = >=TLSv1, <=0305
# Allow only TLS 1.0 and up:
smtpd_tls_protocols = >=0x0301

With Postfix < 3.6 there is no support for a minimum or maximum version,
and the protocol range is configured via protocol exclusions. To re-
quire at least TLS 1.0, set "smtpd_tls_protocols = !SSLv2, !SSLv3".
Listing the protocols to include, rather than protocols to exclude, is
supported, but not recommended. The exclusion form more accurately
matches the underlying OpenSSL interface.

Support for "TLSv1.3" was introduced in OpenSSL 1.1.1. Disabling this
protocol via "!TLSv1.3" is supported since Postfix 3.4 (or patch re-
leases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).

Example:
# Preferred syntax with Postfix >= 3.6:
smtpd_tls_protocols = >=TLSv1, <=TLSv1.3
# Legacy syntax:
smtpd_tls_protocols = !SSLv2, !SSLv3

This feature is available in Postfix 2.6 and later.

smtpd_tls_received_header (default: no)
Request that the Postfix SMTP server produces Received: message headers
that include information about the protocol and cipher used, as well as
the remote SMTP client CommonName and client certificate issuer Common-
Name. This is disabled by default, as the information may be modified
in transit through other mail servers. Only information that was
recorded by the final destination can be trusted.

This feature is available in Postfix 2.2 and later.

smtpd_tls_req_ccert (default: no)
With mandatory TLS encryption, require a trusted remote SMTP client cer-
tificate in order to allow TLS connections to proceed. This option im-
plies "smtpd_tls_ask_ccert = yes".

When TLS encryption is optional, this setting is ignored with a warning
written to the mail log.

This feature is available in Postfix 2.2 and later.

smtpd_tls_security_level (default: empty)
The SMTP TLS security level for the Postfix SMTP server; when a
non-empty value is specified, this overrides the obsolete parameters
smtpd_use_tls and smtpd_enforce_tls. This parameter is ignored with
"smtpd_tls_wrappermode = yes".

Specify one of the following security levels:

none TLS will not be used.

may Opportunistic TLS: announce STARTTLS support to remote SMTP
clients, but do not require that clients use TLS encryption.

encrypt
Mandatory TLS encryption: announce STARTTLS support to remote
SMTP clients, and reject all plaintext commands except HELO,
EHLO, XCLIENT, STARTTLS, NOOP, QUIT, and (Postfix >= 3.9) HELP.
According to RFC 2487 this MUST NOT be applied in case of a pub-
licly-referenced SMTP server. Instead, this should be used on
dedicated servers, for example submission (port 587).

Note 1: the "fingerprint", "verify" and "secure" levels are not sup-
ported here. The Postfix SMTP server logs a warning and uses "encrypt"
instead. To verify remote SMTP client certificates, see TLS_README for
a discussion of the smtpd_tls_ask_ccert, smtpd_tls_req_ccert, and per-
mit_tls_clientcerts features.

Note 2: The parameter setting "smtpd_tls_security_level = encrypt" im-
plies "smtpd_tls_auth_only = yes".

Note 3: when invoked via "sendmail -bs", Postfix will never offer START-
TLS due to insufficient privileges to access the server private key.
This is intended behavior.

This feature is available in Postfix 2.3 and later.

smtpd_tls_session_cache_database (default: empty)
Name of the file containing the optional Postfix SMTP server TLS session
cache. Specify a database type that supports enumeration, such as btree
or sdbm; there is no need to support concurrent access. The file is
created if it does not exist. The smtpd(8) daemon does not use this pa-
rameter directly, rather the cache is implemented indirectly in the
tlsmgr(8) daemon. This means that per-smtpd-instance master.cf overrides
of this parameter are not effective. Note that each of the cache data-
bases supported by tlsmgr(8) daemon: $smtpd_tls_session_cache_database,
$smtp_tls_session_cache_database (and with Postfix 2.3 and later
$lmtp_tls_session_cache_database), needs to be stored separately. It is
not at this time possible to store multiple caches in a single database.

Note: dbm databases are not suitable. TLS session objects are too large.

As of Postfix 2.11 the preferred mechanism for session resumption is RFC
5077 TLS session tickets, which don't require server-side storage. Con-
sequently, for Postfix >= 2.11 this parameter should generally be left
empty. TLS session tickets require an OpenSSL library (at least version
0.9.8h) that provides full support for this TLS extension. See also
smtpd_tls_session_cache_timeout.

Example:

smtpd_tls_session_cache_database = btree:/var/lib/postfix/smtpd_scache

This feature is available in Postfix 2.2 and later.

smtpd_tls_session_cache_timeout (default: 3600s)
The expiration time of Postfix SMTP server TLS session cache informa-
tion. A cache cleanup is performed periodically every $smtpd_tls_ses-
sion_cache_timeout seconds. As with $smtpd_tls_session_cache_database,
this parameter is implemented in the tlsmgr(8) daemon and therefore
per-smtpd-instance master.cf overrides are not possible.

As of Postfix 2.11 this setting cannot exceed 100 days. If set <= 0,
session caching is disabled, not just via the database, but also via RFC
5077 TLS session tickets, which don't require server-side storage. If
set to a positive value less than 2 minutes, the minimum value of 2 min-
utes is used instead. TLS session tickets require an OpenSSL library
(at least version 0.9.8h) that provides full support for this TLS exten-
sion.

This feature is available in Postfix 2.2 and later, and updated for TLS
session ticket support in Postfix 2.11.

smtpd_tls_wrappermode (default: no)
Run the Postfix SMTP server in TLS "wrapper" mode, instead of using the
STARTTLS command.

If you want to support this service, enable a special port in master.cf,
and specify "-o smtpd_tls_wrappermode=yes" on the SMTP server's command
line. Port 465 (submissions, formerly called smtps) is reserved for this
purpose.

This feature is available in Postfix 2.2 and later.

smtpd_upstream_proxy_protocol (default: empty)
The name of the proxy protocol used by an optional before-smtpd proxy
agent. When a proxy agent is used, this protocol conveys local and re-
mote address and port information. Specify "smtpd_upstream_proxy_proto-
col = haproxy" to enable the haproxy protocol; version 2 is supported
with Postfix 3.5 and later.

NOTE: To use the nginx proxy with smtpd(8), enable the XCLIENT protocol
with smtpd_authorized_xclient_hosts. This supports SASL authentication
in the proxy agent (Postfix 2.9 and later).

This feature is available in Postfix 2.10 and later.

smtpd_upstream_proxy_timeout (default: 5s)
The time limit for the proxy protocol specified with the smtpd_up-
stream_proxy_protocol parameter.

This feature is available in Postfix 2.10 and later.

smtpd_use_tls (default: no)
Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but
do not require that clients use TLS encryption.

Note: when invoked via "sendmail -bs", Postfix will never offer STARTTLS
due to insufficient privileges to access the server private key. This is
intended behavior.

This feature is deprecated as of Postfix 3.9. Specify smtpd_tls_secu-
rity_level instead.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and
later use smtpd_tls_security_level instead.

smtputf8_autodetect_classes (default: sendmail, verify)
Detect that a message requires SMTPUTF8 support for the specified mail
origin classes. This is a workaround to avoid chicken-and-egg problems
during the initial SMTPUTF8 roll-out in environments with pre-existing
mail flows that contain UTF8. Those mail flows should not break because
Postfix suddenly refuses to deliver such mail to down-stream MTAs that
don't announce SMTPUTF8 support.

The problem is that Postfix cannot rely solely on the sender's declara-
tion that a message requires SMTPUTF8 support, because UTF8 may be in-
troduced during local processing (for example, the client hostname in
Postfix's Received: header, adding @$myorigin or .$mydomain to an incom-
plete address, address rewriting, alias expansion, automatic BCC recipi-
ents, local forwarding, and changes made by header checks or Milter ap-
plications).

For now, the default is to enable "SMTPUTF8 required" autodetection only
for Postfix sendmail command-line submissions and address verification
probes. This may change once SMTPUTF8 support achieves world domina-
tion. However, sites that add UTF8 content via local processing (see
above) should autodetect the need for SMTPUTF8 support for all email.

Specify one or more of the following:

sendmail
Submission with the Postfix sendmail(1) command.

smtpd Mail received with the smtpd(8) daemon.

qmqpd Mail received with the qmqpd(8) daemon.

forward
Local forwarding or aliasing. When a message is received with
"SMTPUTF8 required", then the forwarded (aliased) message always
has "SMTPUTF8 required".

bounce
Submission by the bounce(8) daemon. When a message is received
with "SMTPUTF8 required", then the delivery status notification
always has "SMTPUTF8 required".

notify
Postmaster notification from the smtp(8) or smtpd(8) daemon.

verify
Address verification probe from the verify(8) daemon.

all Enable SMTPUTF8 autodetection for all mail.

This feature is available in Postfix 3.0 and later.

smtputf8_enable (default: yes)
Enable preliminary SMTPUTF8 support for the protocols described in RFC
6531, RFC 6532, and RFC 6533. This requires that Postfix is built to
support these protocols.

This feature is available in Postfix 3.0 and later.

socketmap_max_reply_size (default: 100000)
The maximum allowed reply size from a socketmap server, not including
the netstring encapsulation.

This feature is available in Postfix >= 3.10.

soft_bounce (default: no)
Safety net to keep mail queued that would otherwise be returned to the
sender. This parameter disables locally-generated bounces, changes the
handling of negative responses from remote servers, content filters or
plugins, and prevents the Postfix SMTP server from rejecting mail perma-
nently by changing 5xx reply codes into 4xx. However, soft_bounce is no
cure for address rewriting mistakes or mail routing mistakes.

Note: "soft_bounce = yes" is in some cases implemented by modifying
server responses. Therefore, the response that Postfix logs may differ
from the response that Postfix actually sends or receives.

Example:

soft_bounce = yes

stale_lock_time (default: 500s)
The time after which a stale exclusive mailbox lockfile is removed.
This is used for delivery to file or mailbox.

stress (default: empty)
This feature is documented in the STRESS_README document.

This feature is available in Postfix 2.5 and later.

strict_7bit_headers (default: no)
Reject mail with 8-bit text in message headers. This blocks mail from
poorly written applications.

This feature should not be enabled on a general purpose mail server, be-
cause it is likely to reject legitimate email.

This feature is available in Postfix 2.0 and later.

strict_8bitmime (default: no)
Enable both strict_7bit_headers and strict_8bitmime_body.

This feature should not be enabled on a general purpose mail server, be-
cause it is likely to reject legitimate email.

This feature is available in Postfix 2.0 and later.

strict_8bitmime_body (default: no)
Reject 8-bit message body text without 8-bit MIME content encoding in-
formation. This blocks mail from poorly written applications.

Unfortunately, this also rejects majordomo approval requests when the
included request contains valid 8-bit MIME mail, and it rejects bounces
from mailers that do not MIME encapsulate 8-bit content (for example,
bounces from qmail or from old versions of Postfix).

This feature should not be enabled on a general purpose mail server, be-
cause it is likely to reject legitimate email.

This feature is available in Postfix 2.0 and later.

strict_mailbox_ownership (default: yes)
Defer delivery when a mailbox file is not owned by its recipient. The
default setting is not backwards compatible.

This feature is available in Postfix 2.5.3 and later.

strict_mime_encoding_domain (default: no)
Reject mail with invalid Content-Transfer-Encoding: information for the
message/* or multipart/* MIME content types. This blocks mail from
poorly written software.

This feature should not be enabled on a general purpose mail server, be-
cause it will reject mail after a single violation.

This feature is available in Postfix 2.0 and later.

strict_rfc821_envelopes (default: no)
Require that addresses received in SMTP MAIL FROM and RCPT TO commands
are enclosed with <>, and that those addresses do not contain RFC 822
style comments or phrases. This stops mail from poorly written soft-
ware.

By default, the Postfix SMTP server accepts RFC 822 syntax in MAIL FROM
and RCPT TO addresses.

strict_smtputf8 (default: no)
Enable stricter enforcement of the SMTPUTF8 protocol. The Postfix SMTP
server accepts UTF8 sender or recipient addresses only when the client
requests an SMTPUTF8 mail transaction.

This feature is available in Postfix 3.0 and later.

sun_mailtool_compatibility (default: no)
Obsolete SUN mailtool compatibility feature. Instead, use "mailbox_de-
livery_lock = dotlock".

swap_bangpath (default: yes)
Enable the rewriting of "site!user" into "user@site". This is necessary
if your machine is connected to UUCP networks. It is enabled by de-
fault.

Note: with Postfix version 2.2, message header address rewriting happens
only when one of the following conditions is true:

• The message is received with the Postfix sendmail(1) command,

• The message is received from a network client that matches $lo-
cal_header_rewrite_clients,

• The message is received from the network, and the re-
mote_header_rewrite_domain parameter specifies a non-empty value.

To get the behavior before Postfix version 2.2, specify "lo-
cal_header_rewrite_clients = static:all".

Example:

swap_bangpath = no

syslog_facility (default: mail)
The syslog facility of Postfix logging. Specify a facility as defined in
syslog.conf(5). The default facility is "mail".

Warning: a non-default syslog_facility setting takes effect only after a
Postfix process has completed initialization. Errors during process
initialization will be logged with the default facility. Examples are
errors while parsing the command line arguments, and errors while ac-
cessing the Postfix main.cf configuration file.

syslog_name (default: see postconf -d output)
A prefix that is prepended to the process name in syslog records, so
that, for example, "smtpd" becomes "prefix/smtpd".

Warning: a non-default syslog_name setting takes effect only after a
Postfix process has completed initialization. Errors during process ini-
tialization will be logged with the default name. Examples are errors
while parsing the command line arguments, and errors while accessing the
Postfix main.cf configuration file.

tcp_windowsize (default: 0)
An optional workaround for routers that break TCP window scaling. Spec-
ify a value > 0 and < 65536 to enable this feature. With Postfix TCP
servers (smtpd(8), qmqpd(8)), this feature is implemented by the Postfix
master(8) daemon.

To change this parameter without stopping Postfix, you need to first
terminate all Postfix TCP servers:

# postconf -e master_service_disable=inet
# postfix reload

This immediately terminates all processes that accept network connec-
tions. Next, you enable Postfix TCP servers with the updated tcp_win-
dowsize setting:

# postconf -e tcp_windowsize=65535 master_service_disable=
# postfix reload

If you skip these steps with a running Postfix system, then the tcp_win-
dowsize change will work only for Postfix TCP clients (smtp(8),
lmtp(8)).

This feature is available in Postfix 2.6 and later.

tls_append_default_CA (default: no)
Append the system-supplied default Certification Authority certificates
to the ones specified with *_tls_CApath or *_tls_CAfile. The default is
"no"; this prevents Postfix from trusting third-party certificates and
giving them relay permission with permit_tls_all_clientcerts.

This feature is available in Postfix 2.4.15, 2.5.11, 2.6.8, 2.7.2 and
later versions. Specify "tls_append_default_CA = yes" for backwards com-
patibility, to avoid breaking certificate verification with sites that
don't use permit_tls_all_clientcerts.

tls_config_file (default: default)
Optional configuration file with baseline OpenSSL settings. OpenSSL
loads any SSL settings found in the configuration file for the selected
application name (see tls_config_name) or else the built-in application
name "openssl_conf" when no application name is specified, or no corre-
sponding configuration section is present.

With OpenSSL releases 1.1.1 and 1.1.1a, applications (including Postfix)
can neither specify an alternative configuration file, nor avoid loading
the default configuration file.

With OpenSSL 1.1.1b or later, this parameter may be set to one of:

default (default)
Load the system-wide "openssl.cnf" configuration file.

none (recommended, OpenSSL 1.1.1b or later only)
This setting disables loading of the system-wide "openssl.cnf"
file.

/absolute-path (OpenSSL 1.1.1b or later only)
Load the configuration file specified by /absolute-path. With
this setting it is an error for the file to not contain any set-
tings for the selected tls_config_name. There is no fallback to
the default "openssl_conf" name.

Failures in processing of the built-in default configuration file, are
silently ignored. Any errors in loading a non-default configuration
file are detected by Postfix, and cause TLS support to be disabled.

The OpenSSL configuration file format is not documented here, beyond
giving two examples.

Example: Default settings for all applications.

# The name 'openssl_conf' is the default application name
# The section name to the right of the '=' sign is arbitrary,
# any name will do, so long as it refers to the desired section.
#
# The name 'system_default' selects the settings applied internally
# by the SSL library as part of SSL object creation. Applications
# can then apply any additional settings of their choice.
#
# In this example, TLS versions prior to 1.2 are disabled by default.
#
openssl_conf = system_wide_settings
[system_wide_settings]
ssl_conf = ssl_library_settings
[ssl_library_settings]
system_default = initial_ssl_settings
[initial_ssl_settings]
MinProtocol = TLSv1.2

Example: Custom settings for an application named "postfix".

# The mapping from an application name to the corresponding configuration
# section must appear near the top of the file, (in what is sometimes called
# the "default section") prior to the start of any explicitly named
# "[sections]". The named sections can appear in any order and don't nest.
#
postfix = postfix_settings
[postfix_settings]
ssl_conf = postfix_ssl_settings
[postfix_ssl_settings]
system_default = baseline_postfix_settings
[baseline_postfix_settings]
MinProtocol = TLSv1

Example: Custom OpenSSL group settings.

main.cf:
tls_config_file = ${config_directory}/openssl.cnf
tls_config_name = postfix

openssl.cnf:
postfix = postfix_settings

[postfix_settings]
ssl_conf = postfix_ssl_settings

[postfix_ssl_settings]
system_default = baseline_postfix_settings

[baseline_postfix_settings]
# New OpenSSL 3.5 syntax, for older releases consider
# the Postfix default:
#
# Groups = X25519:X448:prime256v1:secp384r1:secp521r1:ffdhe2048:ffdhe3072
#
Groups = *X25519MLKEM768 / *X25519:X448 / P-256:P-384

Caution: It is typically best to just use the default OpenSSL group set-
tings, by setting "tls_config_file = none". Overly strict system-wide
TLS settings will conflict with Postfix's opportunistic TLS, where being
less restrictive is better than downgrading to cleartext SMTP.

This feature is available in Postfix >= 3.9, 3.8.1, 3.7.6, 3.6.10, and
3.5.20.

tls_config_name (default: empty)
The application name passed by Postfix to OpenSSL library initialization
functions. This name is used to select the desired configuration "sec-
tion" in the OpenSSL configuration file specified via the tls_con-
fig_file parameter. When empty, or when the selected name is not
present in the configuration file, the default application name
("openssl_conf") is used as a fallback.

This feature is available in Postfix >= 3.9.

tls_daemon_random_bytes (default: 32)
The number of pseudo-random bytes that an smtp(8) or smtpd(8) process
requests from the tlsmgr(8) server in order to seed its internal pseudo
random number generator (PRNG). The default of 32 bytes (equivalent to
256 bits) is sufficient to generate a 128bit (or 168bit) session key.

This feature is available in Postfix 2.2 and later.

tls_dane_digest_agility (default: on)
Configure RFC7671 DANE TLSA digest algorithm agility. Do not change
this setting from its default value.

See Section 8 of RFC7671 for correct key rotation procedures.

This feature is available in Postfix 2.11 through 3.1. Postfix 3.2 and
later ignore this configuration parameter and behave as though it were
set to "on".

tls_dane_digests (default: sha512 sha256)
DANE TLSA (RFC 6698, RFC 7671, RFC 7672) resource-record "matching type"
digest algorithms in descending preference order. All the specified al-
gorithms must be supported by the underlying OpenSSL library, otherwise
the Postfix SMTP client will not support DANE TLSA security.

Specify a list of digest names separated by commas and/or whitespace.
Each digest name may be followed by an optional "=<number>" suffix. For
example, "sha512" may instead be specified as "sha512=2" and "sha256"
may instead be specified as "sha256=1". The optional number must match
the <a href="https://www.iana.org/assignments/dane-parameters/dane-para-
meters.xhtml#matching-types" >IANA assigned TLSA matching type number
the algorithm in question. Postfix will check this constraint for the
algorithms it knows about. Additional matching type algorithms regis-
tered with IANA can be added with explicit numbers provided they are
supported by OpenSSL.

Invalid list elements are logged with a warning and disable DANE sup-
port. TLSA RRs that specify digests not included in the list are ig-
nored with a warning.

Note: It is unwise to omit sha256 from the digest list. This digest al-
gorithm is the only mandatory to implement digest algorithm in RFC 6698,
and many servers are expected to publish TLSA records with just sha256
digests. Unless one of the standard digests is seriously compromised
and servers have had ample time to update their TLSA records you should
not omit any standard digests, just arrange them in order from strongest
to weakest.

This feature is available in Postfix 2.11 and later.

tls_dane_trust_anchor_digest_enable (default: yes)
Enable support for RFC 6698 (DANE TLSA) DNS records that contain digests
of trust-anchors with certificate usage "2". Do not change this setting
from its default value.

This feature is available in Postfix 2.11 through 3.1. It has been
withdrawn in Postfix 3.2, as trust-anchor TLSA records are now widely
used and have proved sufficiently reliable. Postfix 3.2 and later ig-
nore this configuration parameter and behaves as though it were set to
"yes".

tls_disable_workarounds (default: see postconf -d output)
List or bit-mask of OpenSSL bug work-arounds to disable.

The OpenSSL toolkit includes a set of work-arounds for buggy SSL/TLS im-
plementations. Applications, such as Postfix, that want to maximize in-
teroperability ask the OpenSSL library to enable the full set of recom-
mended work-arounds.

From time to time, it is discovered that a work-around creates a secu-
rity issue, and should no longer be used. If upgrading OpenSSL to a
fixed version is not an option or an upgrade is not available in a
timely manner, or in closed environments where no buggy clients or
servers exist, it may be appropriate to disable some or all of the
OpenSSL interoperability work-arounds. This parameter specifies which
bug work-arounds to disable.

If the value of the parameter is a hexadecimal long integer starting
with "0x", the bug work-arounds corresponding to the bits specified in
its value are removed from the SSL_OP_ALL work-around bit-mask (see
openssl/ssl.h and SSL_CTX_set_options(3)). You can specify more bits
than are present in SSL_OP_ALL, excess bits are ignored. Specifying
0xFFFFFFFF disables all bug-workarounds on a 32-bit system. This should
also be sufficient on 64-bit systems, until OpenSSL abandons support for
32-bit systems and starts using the high 32 bits of a 64-bit
bug-workaround mask.

Otherwise, the parameter is a white-space or comma separated list of
specific named bug work-arounds chosen from the list below. It is possi-
ble that your OpenSSL version includes new bug work-arounds added after
your Postfix source code was last updated, in that case you can only
disable one of these via the hexadecimal syntax above.

CRYPTOPRO_TLSEXT_BUG
New with GOST support in OpenSSL 1.0.0.

DONT_INSERT_EMPTY_FRAGMENTS
See SSL_CTX_set_options(3)

LEGACY_SERVER_CONNECT
See SSL_CTX_set_options(3)

MICROSOFT_BIG_SSLV3_BUFFER
See SSL_CTX_set_options(3)

MICROSOFT_SESS_ID_BUG
See SSL_CTX_set_options(3)

MSIE_SSLV2_RSA_PADDING
also aliased as CVE-2005-2969. Postfix 2.8 disables this
work-around by default with OpenSSL versions that may predate the
fix. Fixed in OpenSSL 0.9.7h and OpenSSL 0.9.8a.

NETSCAPE_CHALLENGE_BUG
See SSL_CTX_set_options(3)

NETSCAPE_REUSE_CIPHER_CHANGE_BUG
also aliased as CVE-2010-4180. Postfix 2.8 disables this
work-around by default with OpenSSL versions that may predate the
fix. Fixed in OpenSSL 0.9.8q and OpenSSL 1.0.0c.

SSLEAY_080_CLIENT_DH_BUG
See SSL_CTX_set_options(3)

SSLREF2_REUSE_CERT_TYPE_BUG
See SSL_CTX_set_options(3)

TLS_BLOCK_PADDING_BUG
See SSL_CTX_set_options(3)

TLS_D5_BUG
See SSL_CTX_set_options(3)

TLS_ROLLBACK_BUG
See SSL_CTX_set_options(3). This is disabled in OpenSSL 0.9.7
and later. Nobody should still be using 0.9.6!

TLSEXT_PADDING
Postfix >= 3.4. See SSL_CTX_set_options(3).

This feature is available in Postfix 2.8 and later.

tls_eecdh_auto_curves (default: see postconf -d output)
The prioritized list of elliptic curves, that should be enabled in the
Postfix SMTP client and server. The selected curves should be imple-
mented by OpenSSL and be standardized for use in the TLS "supported
groups" extension (RFC8422, RFC8446 and RFC8447). Be sure to include at
least "x25519" and "prime256v1" (the OpenSSL name for "secp256r1",
a.k.a. "P-256"). The default list is suitable for most users.

On the client side, the first curve listed will be used to construct the
client's initial TLS 1.3 "keyshare". If this is not supported by the
server, the TLS handshake may require an additional round-trip after the
server issues a HelloRetryRequest (HRR) indicating a suitable mutually
supported curve.

Postfix skips curve names that are unknown to OpenSSL, or that are known
but not yet implemented. This makes it possible to "anticipate" support
for curves that should be used once they become available, or to deploy
the same setting on a server "farm" where not all servers support the
same curves.

As of Postfix 3.10, when compiled with OpenSSL 3.0 or later, the "curve"
names can be more general key encapsulation mechanisms (KEMs), and/or
may be loaded from an external "provider" (via a suitable tls_con-
fig_file).

See also the "tls_ffdhe_auto_groups" parameter, which supports customiz-
ing the list of FFDHE groups enabled with TLS 1.3. That setting is in-
troduced with Postfix 3.8, when built against OpenSSL 3.0 or later.

Post-quantum cryptography support: OpenSSL 3.5 introduces new configura-
tion syntax that Postfix will not attempt to imitate. Instead, with
Postfix 3.6.17, 3.7.13, 3.8.8, 3.9.2, and later, set both
tls_eecdh_auto_curves and if available tls_ffdhe_auto_groups to the
empty value, to enable algorithm selection through OpenSSL configura-
tion. See tls_config_file for a configuration example.

This feature is available in Postfix 3.2 and later, when it is compiled
and linked with OpenSSL 1.0.2 or later on platforms where EC algorithms
have not been disabled by the vendor.

tls_eecdh_strong_curve (default: prime256v1)
The elliptic curve used by the Postfix SMTP server for sensibly strong
ephemeral ECDH key exchange. This curve is used by the Postfix SMTP
server when "smtpd_tls_eecdh_grade = strong". The phrase "sensibly
strong" means approximately 128-bit security based on best known at-
tacks. The selected curve must be implemented by OpenSSL (as reported by
ecparam(1) with the "-list_curves" option) and be one of the curves
listed in Section 5.1.1 of RFC 8422. You should not generally change
this setting. Remote SMTP client implementations must support this
curve for EECDH key exchange to take place. It is unwise to choose only
"bleeding-edge" curves supported by only a small subset of clients.

This feature is not used as of Postfix 3.6. Do not specify.

The default "strong" curve is rated in NSA Suite B for information clas-
sified up to SECRET.

Note: elliptic curve names are poorly standardized; different standards
groups are assigning different names to the same underlying curves. The
curve with the X9.62 name "prime256v1" is also known under the SECG name
"secp256r1", but OpenSSL does not recognize the latter name.

This feature is available in Postfix 2.6 and later, when it is compiled
and linked with OpenSSL 1.0.0 or later on platforms where EC algorithms
have not been disabled by the vendor.

tls_eecdh_ultra_curve (default: secp384r1)
The elliptic curve used by the Postfix SMTP server for maximally strong
ephemeral ECDH key exchange. This curve is used by the Postfix SMTP
server when "smtpd_tls_eecdh_grade = ultra". The phrase "maximally
strong" means approximately 192-bit security based on best known at-
tacks. This additional strength comes at a significant computational
cost, most users should instead set "smtpd_tls_eecdh_grade = strong".
The selected curve must be implemented by OpenSSL (as reported by ec-
param(1) with the "-list_curves" option) and be one of the curves listed
in Section 5.1.1 of RFC 8422. You should not generally change this set-
ting. Remote SMTP client implementations must support this curve for
EECDH key exchange to take place. It is unwise to choose only "bleed-
ing-edge" curves supported by only a small subset of clients.

This feature is not used as of Postfix 3.6. Do not specify.

This default "ultra" curve is rated in NSA Suite B for information clas-
sified up to TOP SECRET.

This feature is available in Postfix 2.6 and later, when it is compiled
and linked with OpenSSL 1.0.0 or later on platforms where EC algorithms
have not been disabled by the vendor.

tls_export_cipherlist (default: see postconf -d output)
The OpenSSL cipherlist for "export" or higher grade ciphers. Ignored as
of Postfix 3.8. In earlier Postfix releases this defined the meaning of
the "export" setting in smtpd_tls_ciphers, smtpd_tls_mandatory_ciphers,
smtp_tls_ciphers, smtp_tls_mandatory_ciphers, lmtp_tls_ciphers, and
lmtp_tls_mandatory_ciphers. You are strongly encouraged not to change
this setting.

This feature is available in Postfix 2.3 and later.

tls_fast_shutdown_enable (default: yes)
A workaround for implementations that hang Postfix while shutting down a
TLS session, until Postfix times out. With this enabled, Postfix will
not wait for the remote TLS peer to respond to a TLS 'close' notifica-
tion. This behavior is recommended for TLSv1.0 and later.

tls_ffdhe_auto_groups (default: see postconf -d output)
The prioritized list of finite-field Diffie-Hellman ephemeral (FFDHE)
key exchange groups supported by the Postfix SMTP client and server.
OpenSSL 3.0 adds support for FFDHE key agreement in TLS 1.3. In OpenSSL
1.1.1, TLS 1.3 was only supported with elliptic-curve based key agree-
ment. The "tls_ffdhe_auto_groups" parameter makes it possible to con-
figure the list of FFDHE groups that the Postfix client or server will
enable in OpenSSL 3.0 and up. This parameter has no effect when Postfix
is built against earlier OpenSSL versions.

The default list of FFDHE groups that Postfix enables in OpenSSL 3.0 and
up includes just the 2048 and 3072-bit groups. Stronger FFDHE groups
perform poorly and EC groups are a much better choice for the same secu-
rity level. Postfix ignores group names that are unknown to OpenSSL, or
that are known but not yet implemented. The FFDHE groups are largely a
backup, in case some peer does not support EC key exchange, or EC key
exchange needs to be disabled for some pressing reason.

Setting this parameter empty disables FFDHE support in TLS 1.3. Whether
FFDHE key agreement is enabled in TLS 1.2 and earlier depends on whether
any of the "kDHE" ciphers are included in the cipherlist.

Conversely, setting "tls_eecdh_auto_curves" empty disables TLS 1.3 EC
key agreement in OpenSSL 3.0 and later. If both are set empty, Postfix
will fall back to OpenSSL preferences as described next.

Post-quantum cryptography support: OpenSSL 3.5 introduces new configura-
tion syntax that Postfix will not attempt to imitate. Instead, with
Postfix 3.6.17, 3.7.13, 3.8.8, 3.9.2, and later, set both
tls_eecdh_auto_curves and tls_ffdhe_auto_groups to the empty value, to
enable algorithm selection through OpenSSL configuration. See tls_con-
fig_file for a configuration example.

All the default groups and EC curves should be sufficiently strong to
make "pruning" the defaults unwise. At a minimum, "x25519" and
"prime256v1" (the OpenSSL name for "secp256r1", a.k.a. "P-256") should
be among the enabled EC curves, while "dhe2048" and "dhe3072" should be
among the FFDHE groups.

This feature is available in Postfix 3.8 and later, when it is compiled
and linked with OpenSSL 3.0 or later.

tls_high_cipherlist (default: see postconf -d output)
The OpenSSL cipherlist for "high" grade ciphers. This defines the mean-
ing of the "high" setting in smtpd_tls_ciphers, smtpd_tls_mandatory_ci-
phers, smtp_tls_ciphers, smtp_tls_mandatory_ciphers, lmtp_tls_ciphers,
and lmtp_tls_mandatory_ciphers. You are strongly encouraged not to
change this setting.

This feature is available in Postfix 2.3 and later.

tls_legacy_public_key_fingerprints (default: no)
A temporary migration aid for sites that use certificate public-key fin-
gerprints with Postfix 2.9.0..2.9.5, which use an incorrect algorithm.
This parameter has no effect on the certificate fingerprint support that
is available since Postfix 2.2.

Specify "tls_legacy_public_key_fingerprints = yes" temporarily, pending
a migration from configuration files with incorrect Postfix 2.9.0..2.9.5
certificate public-key finger prints, to the correct fingerprints used
by Postfix 2.9.6 and later. To compute the correct certificate pub-
lic-key fingerprints, see TLS_README.

This feature is available in Postfix 2.9.6 and later.

tls_low_cipherlist (default: see postconf -d output)
The OpenSSL cipherlist for "low" or higher grade ciphers. Ignored as of
Postfix 3.8. In earlier Postfix releases this defined the meaning of
the "low" setting in smtpd_tls_ciphers, smtpd_tls_mandatory_ciphers,
smtp_tls_ciphers, smtp_tls_mandatory_ciphers, lmtp_tls_ciphers, and
lmtp_tls_mandatory_ciphers. You are strongly encouraged not to change
this setting.

This feature is available in Postfix 2.3 and later.

tls_medium_cipherlist (default: see postconf -d output)
The OpenSSL cipherlist for "medium" or higher grade ciphers. This de-
fines the meaning of the "medium" setting in smtpd_tls_ciphers,
smtpd_tls_mandatory_ciphers, smtp_tls_ciphers, smtp_tls_mandatory_ci-
phers, lmtp_tls_ciphers, and lmtp_tls_mandatory_ciphers. This is the
default cipherlist for mandatory TLS encryption in the TLS client (with
anonymous ciphers disabled when verifying server certificates). This is
the default cipherlist for opportunistic TLS with Postfix releases after
the middle of 2015. You are strongly encouraged not to change this set-
ting.

This feature is available in Postfix 2.3 and later.

tls_null_cipherlist (default: eNULL:!aNULL)
The OpenSSL cipherlist for "NULL" grade ciphers that provide authentica-
tion without encryption. This defines the meaning of the "null" setting
in smtpd_tls_mandatory_ciphers, smtp_tls_mandatory_ciphers and
lmtp_tls_mandatory_ciphers. You are strongly encouraged not to change
this setting.

This feature is available in Postfix 2.3 and later.

tls_preempt_cipherlist (default: no)
With SSLv3 and later, use the Postfix SMTP server's cipher preference
order instead of the remote client's cipher preference order.

By default, the OpenSSL server selects the client's most preferred ci-
pher that the server supports. With SSLv3 and later, the server may
choose its own most preferred cipher that is supported (offered) by the
client. Setting "tls_preempt_cipherlist = yes" enables server cipher
preferences.

While server cipher selection may in some cases lead to a more secure or
performant cipher choice, there is some risk of interoperability issues.
In the past, some SSL clients have listed lower priority ciphers that
they did not implement correctly. If the server chooses a cipher that
the client prefers less, it may select a cipher whose client implementa-
tion is flawed. Most notably Windows 2003 Microsoft Exchange servers
have flawed implementations of DES-CBC3-SHA, which OpenSSL considers
stronger than RC4-SHA. Enabling server cipher-suite selection may cre-
ate interoperability issues with Windows 2003 Microsoft Exchange
clients.

This feature is available in Postfix 2.8 and later, in combination with
OpenSSL 0.9.7 and later.

tls_random_bytes (default: 32)
The number of bytes that tlsmgr(8) reads from $tls_random_source when
(re)seeding the in-memory pseudo random number generator (PRNG) pool.
The default of 32 bytes (256 bits) is good enough for 128bit symmetric
keys. If using EGD or a device file, a maximum of 255 bytes is read.

This feature is available in Postfix 2.2 and later.

tls_random_exchange_name (default: see postconf -d output)
Name of the pseudo random number generator (PRNG) state file that is
maintained by tlsmgr(8). The file is created when it does not exist, and
its length is fixed at 1024 bytes.

As of version 2.5, Postfix no longer uses root privileges when opening
this file, and the default file location was changed from ${config_di-
rectory}/prng_exch to ${data_directory}/prng_exch. As a migration aid,
an attempt to open the file under a non-Postfix directory is redirected
to the Postfix-owned data_directory, and a warning is logged.

This feature is available in Postfix 2.2 and later.

tls_random_prng_update_period (default: 3600s)
The time between attempts by tlsmgr(8) to save the state of the pseudo
random number generator (PRNG) to the file specified with $tls_ran-
dom_exchange_name.

This feature is available in Postfix 2.2 and later.

tls_random_reseed_period (default: 3600s)
The maximal time between attempts by tlsmgr(8) to re-seed the in-memory
pseudo random number generator (PRNG) pool from external sources. The
actual time between re-seeding attempts is calculated using the PRNG,
and is between 0 and the time specified.

This feature is available in Postfix 2.2 and later.

tls_random_source (default: see postconf -d output)
The external entropy source for the in-memory tlsmgr(8) pseudo random
number generator (PRNG) pool. Be sure to specify a non-blocking source.
If this source is not a regular file, the entropy source type must be
prepended: egd:/path/to/egd_socket for a source with EGD compatible
socket interface, or dev:/path/to/device for a device file.

Note: on OpenBSD systems specify dev:/dev/arandom when dev:/dev/urandom
gives timeout errors.

This feature is available in Postfix 2.2 and later.

tls_required_enable (default: yes)
Enable support for the "TLS-Required: no" message header, defined in RFC
8689. By adding this header to a message, a sender requests no enforce-
ment of TLS policy. This limits the Postfix SMTP client TLS security
level to "may", that is, do not verify remote SMTP server certificates,
and fall back to plaintext if TLS is unavailable. If a message contains
a "TLS-Required: no" header, then Postfix will add that header to a de-
livery status notification for that message.

This feature is available in Postfix >= 3.10.

tls_server_sni_maps (default: empty)
Optional lookup tables that map names received from remote SMTP clients
via the TLS Server Name Indication (SNI) extension to the appropriate
keys and certificate chains. This parameter is implemented in the Post-
fix TLS library, and applies to both smtpd(8) and the SMTP server mode
of tlsproxy(8).

When this parameter is non-empty, the Postfix SMTP server enables SNI
extension processing, and logs SNI values that are invalid or don't
match an entry in the specified tables. When an entry does match, the
SNI name is logged as part of the connection summary at log levels 1 and
higher.

The lookup key is either the verbatim SNI domain name or an ancestor do-
main prefixed with a leading dot. For internationalized domains, the
lookup key must be in IDNA 2008 A-label form (as required in the TLS SNI
extension).

The syntax of the lookup value is the same as with the
smtp_tls_chain_files parameter (see there for additional details), but
here scoped to just TLS connections in which the client sends a matching
SNI domain name.

Example:

/etc/postfix/main.cf:
#
# The indexed SNI table must be created with "postmap -F"
#
indexed = ${default_database_type}:${config_directory}/
tls_server_sni_maps = ${indexed}sni

/etc/postfix/sni:
#
# The example.com domain has both an RSA and ECDSA certificate
# chain. The chain files MUST start with the private key,
# with the certificate chain next, starting with the leaf
# (server) certificate, and then the issuer certificates.
#
example.com /etc/postfix/sni-chains/rsa2048.example.com.pem,
/etc/postfix/sni-chains/ecdsa-p256.example.com.pem
#
# The example.net domain has a wildcard certificate, and two
# additional DNS names. So its certificate chain is also used
# with any subdomain, plus the additional names.
#
example.net /etc/postfix/sni-chains/example.net.pem
.example.net /etc/postfix/sni-chains/example.net.pem
example.info /etc/postfix/sni-chains/example.net.pem
example.org /etc/postfix/sni-chains/example.net.pem

Note that the SNI lookup tables should also have entries for the domains
that correspond to the Postfix SMTP server's default certificate(s).
This ensures that the remote SMTP client's TLS SNI extension gets a pos-
itive response when it specifies one of the Postfix SMTP server's de-
fault domains, and ensures that the Postfix SMTP server will not log an
SNI name mismatch for such a domain. The Postfix SMTP server's default
certificates are then only used when the client sends no SNI or when it
sends SNI with a domain that the server knows no certificate(s) for.

The mapping from an SNI domain name to a certificate chain is indirect.
In the input source files for "cdb", "hash", "btree" or other tables
that are converted to on-disk indexed files via postmap(1), the value
specified for each key is a list of filenames. When postmap(1) is used
with the -F option, the generated table stores for each lookup key the
base64-encoded contents of the associated files. When querying tables
via postmap -Fq, the table value is decoded from base64, yielding the
original file content, plus a new line.

With "regexp", "pcre", "inline", "texthash", "static" and similar tables
that are interpreted at run-time, and don't have a separate source for-
mat, the table value is again a list files, that are loaded into memory
when the table is opened.

With tables whose content is managed outside of Postfix, such as LDAP,
MySQL, PostgreSQL, socketmap and tcp, the value must be a concatenation
of the desired PEM keys and certificate chains, that is then further en-
coded to yield a single-line base64 string. Creation of such tables and
secure storage (the value includes private key material) are outside the
responsibility of Postfix.

With "socketmap" and "tcp" the data will be transmitted in the clear,
and there is no query access control, so these are generally unsuitable
for storing SNI chains. With LDAP and SQL, you should restrict read ac-
cess and use TLS to protect the sensitive data in transit.

Typically there is only one private key and its chain of certificates
starting with the "leaf" certificate corresponding to that key, and con-
tinuing with the appropriate intermediate issuer CA certificates, with
each certificate ideally followed by its issuer. Servers that have keys
and certificates for more than one algorithm (e.g. both an RSA key and
an ECDSA key, or even RSA, ECDSA and Ed25519) can use multiple chains
concatenated together, with the key always listed before the correspond-
ing certificates.

This feature is available in Postfix 3.4 and later.

tls_session_ticket_cipher (default: Postfix >= 3.0: aes-256-cbc, Postfix < 3.0:
aes-128-cbc)
Algorithm used to encrypt RFC5077 TLS session tickets. This algorithm
must use CBC mode, have a 128-bit block size, and must have a key length
between 128 and 256 bits. The default is aes-256-cbc. Overriding the
default to choose a different algorithm is discouraged.

Setting this parameter empty disables session ticket support in the
Postfix SMTP server. Another way to disable session ticket support is
via the tls_ssl_options parameter.

This feature is available in Postfix 3.0 and later.

tls_ssl_options (default: empty)
List or bit-mask of OpenSSL options to enable.

The OpenSSL toolkit provides a set of options that applications can en-
able to tune the OpenSSL behavior. Some of these work around bugs in
other implementations and are on by default. You can use the tls_dis-
able_workarounds parameter to selectively disable some or all of the bug
work-arounds, making OpenSSL more strict at the cost of non-interoper-
ability with SSL clients or servers that exhibit the bugs.

Other options are off by default, and typically enable or disable fea-
tures rather than bug work-arounds. These may be turned on (with care)
via the tls_ssl_options parameter. The value is a white-space or comma
separated list of named options chosen from the list below. The names
are not case-sensitive, you can use lower-case if you prefer. The upper
case values below match the corresponding macro name in the ssl.h header
file with the SSL_OP_ prefix removed. It is possible that your OpenSSL
version includes new options added after your Postfix source code was
last updated, in that case you can only enable one of these via the
hexadecimal syntax below.

You should only enable features via the hexadecimal mask when the need
to control the feature is critical (to deal with a new vulnerability or
a serious interoperability problem). Postfix DOES NOT promise backwards
compatible behavior with respect to the mask bits. A feature enabled
via the mask in one release may be enabled by other means in a later re-
lease, and the mask bit will then be ignored. Therefore, use of the
hexadecimal mask is only a temporary measure until a new Postfix or
OpenSSL release provides a better solution.

If the value of the parameter is a hexadecimal long integer starting
with "0x", the options corresponding to the bits specified in its value
are enabled (see openssl/ssl.h and SSL_CTX_set_options(3)). You can
only enable options not already controlled by other Postfix settings.
For example, you cannot disable protocols or enable server cipher pref-
erence. Do not attempt to enable all features by specifying 0xFFFFFFFF,
this is unlikely to be a good idea. Some bug work-arounds are also
valid here, allowing them to be re-enabled if/when they're no longer en-
abled by default. The supported values include:

ENABLE_MIDDLEBOX_COMPAT
Postfix >= 3.4. See SSL_CTX_set_options(3).

LEGACY_SERVER_CONNECT
See SSL_CTX_set_options(3).

NO_TICKET
Enabled by default when needed in fully-patched Postfix >= 2.7.
Not needed at all for Postfix >= 2.11, unless for some reason you
do not want to support TLS session resumption. Best not set ex-
plicitly. See SSL_CTX_set_options(3).

NO_COMPRESSION
Disable SSL compression even if supported by the OpenSSL library.
Compression is CPU-intensive, and compression before encryption
does not always improve security.

NO_RENEGOTIATION
Postfix >= 3.4. This can reduce opportunities for a potential
CPU exhaustion attack. See SSL_CTX_set_options(3).

NO_SESSION_RESUMPTION_ON_RENEGOTIATION
Postfix >= 3.4. See SSL_CTX_set_options(3).

PRIORITIZE_CHACHA
Postfix >= 3.4. See SSL_CTX_set_options(3).

This feature is available in Postfix 2.11 and later.

tls_wildcard_matches_multiple_labels (default: yes)
Match multiple DNS labels with "*" in wildcard certificates.

Some mail service providers prepend the customer domain name to a base
domain for which they have a wildcard TLS certificate. For example, the
MX records for example.com hosted by example.net may be:

example.com. IN MX 0 example.com.mx1.example.net.
example.com. IN MX 0 example.com.mx2.example.net.

and the TLS certificate may be for "*.example.net". The "*" then corre-
sponds with multiple labels in the mail server domain name. While
multi-label wildcards are not widely supported, and are not blessed by
any standard, there is little to be gained by disallowing their use in
this context.

Notes:

• In a certificate name, the "*" is special only when it is used as
the first label.

• While Postfix (2.11 or later) can match "*" with multiple domain
name labels, other implementations likely will not.

• Earlier Postfix implementations behave as if "tls_wild-
card_matches_multiple_labels = no".

This feature is available in Postfix 2.11 and later.

tlsmgr_service_name (default: tlsmgr)
The name of the tlsmgr(8) service entry in master.cf. This service main-
tains TLS session caches and other information in support of TLS.

This feature is available in Postfix 2.11 and later.

tlsproxy_client_CAfile (default: $smtp_tls_CAfile)
A file containing CA certificates of root CAs trusted to sign either re-
mote TLS server certificates or intermediate CA certificates. See
smtp_tls_CAfile for further details.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_CApath (default: $smtp_tls_CApath)
Directory with PEM format Certification Authority certificates that the
Postfix tlsproxy(8) client uses to verify a remote TLS server certifi-
cate. See smtp_tls_CApath for further details.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_cert_file (default: $smtp_tls_cert_file)
File with the Postfix tlsproxy(8) client RSA certificate in PEM format.
See smtp_tls_cert_file for further details. The preferred way to con-
figure tlsproxy client keys and certificates is via the "tl-
sproxy_client_chain_files" parameter.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_chain_files (default: $smtp_tls_chain_files)
Files with the Postfix tlsproxy(8) client keys and certificate chains in
PEM format. See smtp_tls_chain_files for further details.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_dcert_file (default: $smtp_tls_dcert_file)
File with the Postfix tlsproxy(8) client DSA certificate in PEM format.
See smtp_tls_dcert_file for further details. DSA is obsolete and should
not be used.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_dkey_file (default: $smtp_tls_dkey_file)
File with the Postfix tlsproxy(8) client DSA private key in PEM format.
See smtp_tls_dkey_file for further details. DSA is obsolete and should
not be used.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_eccert_file (default: $smtp_tls_eccert_file)
File with the Postfix tlsproxy(8) client ECDSA certificate in PEM for-
mat. See smtp_tls_eccert_file for further details. The preferred way to
configure tlsproxy client keys and certificates is via the "tl-
sproxy_client_chain_files" parameter.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_eckey_file (default: $smtp_tls_eckey_file)
File with the Postfix tlsproxy(8) client ECDSA private key in PEM for-
mat. See smtp_tls_eckey_file for further details. The preferred way to
configure tlsproxy client keys and certificates is via the "tl-
sproxy_client_chain_files" parameter.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_enforce_tls (default: $smtp_enforce_tls)
Enforcement mode: require that SMTP servers use TLS encryption. See
smtp_enforce_tls for further details. Use tlsproxy_client_security_level
instead.

This feature is deprecated as of Postfix 3.9. Specify tl-
sproxy_client_security_level instead.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_fingerprint_digest (default: $smtp_tls_fingerprint_digest)
The message digest algorithm used to construct remote TLS server cer-
tificate fingerprints. See smtp_tls_fingerprint_digest for further de-
tails.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_key_file (default: $smtp_tls_key_file)
File with the Postfix tlsproxy(8) client RSA private key in PEM format.
See smtp_tls_key_file for further details. The preferred way to config-
ure tlsproxy client keys and certificates is via the "tl-
sproxy_client_chain_files" parameter.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_level (default: $smtp_tls_security_level)
The default TLS security level for the Postfix tlsproxy(8) client. See
smtp_tls_security_level for further details.

This feature is available in Postfix 3.4 - 3.6. It was renamed to tl-
sproxy_client_security_level in Postfix 3.7.

tlsproxy_client_loglevel (default: $smtp_tls_loglevel)
Enable additional Postfix tlsproxy(8) client logging of TLS activity.
See smtp_tls_loglevel for further details.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_loglevel_parameter (default: smtp_tls_loglevel)
The name of the parameter that provides the tlsproxy_client_loglevel
value.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_per_site (default: $smtp_tls_per_site)
Optional lookup tables with the Postfix tlsproxy(8) client TLS usage
policy by next-hop destination and by remote TLS server hostname. See
smtp_tls_per_site for further details.

This feature is deprecated as of Postfix 3.9. Specify tl-
sproxy_client_policy_maps instead.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_policy (default: $smtp_tls_policy_maps)
Optional lookup tables with the Postfix tlsproxy(8) client TLS security
policy by next-hop destination. See smtp_tls_policy_maps for further de-
tails.

This feature is available in Postfix 3.4 - 3.6. It was renamed to tl-
sproxy_client_policy_maps in Postfix 3.7.

tlsproxy_client_policy_maps (default: $smtp_tls_policy_maps)
Optional lookup tables with the Postfix tlsproxy(8) client TLS security
policy by next-hop destination. See smtp_tls_policy_maps for further de-
tails.

This feature is available in Postfix 3.7 and later. It was previously
called tlsproxy_client_policy.

tlsproxy_client_scert_verifydepth (default: $smtp_tls_scert_verifydepth)
The verification depth for remote TLS server certificates. See
smtp_tls_scert_verifydepth for further details.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_security_level (default: $smtp_tls_security_level)
The default TLS security level for the Postfix tlsproxy(8) client. See
smtp_tls_security_level for further details.

This feature is available in Postfix 3.7 and later. It was previously
called tlsproxy_client_level.

tlsproxy_client_use_tls (default: $smtp_use_tls)
Opportunistic mode: use TLS when a remote server announces TLS support.
See smtp_use_tls for further details. Use tlsproxy_client_security_level
instead.

This feature is deprecated as of Postfix 3.9. Specify tl-
sproxy_client_security_level instead.

This feature is available in Postfix 3.4 and later.

tlsproxy_enforce_tls (default: $smtpd_enforce_tls)
Mandatory TLS: announce STARTTLS support to remote SMTP clients, and re-
quire that clients use TLS encryption. See smtpd_enforce_tls for further
details. Use tlsproxy_tls_security_level instead.

This feature is deprecated as of Postfix 3.9. Specify tlsproxy_tls_secu-
rity_level instead.

This feature is available in Postfix 2.8 and later.

tlsproxy_service_name (default: tlsproxy)
The name of the tlsproxy(8) service entry in master.cf. This service
performs plaintext <=> TLS ciphertext conversion.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_CAfile (default: $smtpd_tls_CAfile)
A file containing (PEM format) CA certificates of root CAs trusted to
sign either remote SMTP client certificates or intermediate CA certifi-
cates. See smtpd_tls_CAfile for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_CApath (default: $smtpd_tls_CApath)
A directory containing (PEM format) CA certificates of root CAs trusted
to sign either remote SMTP client certificates or intermediate CA cer-
tificates. See smtpd_tls_CApath for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_always_issue_session_ids (default: $smtpd_tls_always_issue_ses-
sion_ids)
Force the Postfix tlsproxy(8) server to issue a TLS session id, even
when TLS session caching is turned off. See smtpd_tls_always_issue_ses-
sion_ids for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_ask_ccert (default: $smtpd_tls_ask_ccert)
Ask a remote SMTP client for a client certificate. See
smtpd_tls_ask_ccert for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_ccert_verifydepth (default: $smtpd_tls_ccert_verifydepth)
The verification depth for remote SMTP client certificates. A depth of 1
is sufficient if the issuing CA is listed in a local CA file. See
smtpd_tls_ccert_verifydepth for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_cert_file (default: $smtpd_tls_cert_file)
File with the Postfix tlsproxy(8) server RSA certificate in PEM format.
This file may also contain the Postfix tlsproxy(8) server private RSA
key. See smtpd_tls_cert_file for further details. With Postfix >= 3.4
the preferred way to configure tlsproxy server keys and certificates is
via the "tlsproxy_tls_chain_files" parameter.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_chain_files (default: $smtpd_tls_chain_files)
Files with the Postfix tlsproxy(8) server keys and certificate chains in
PEM format. See smtpd_tls_chain_files for further details.

This feature is available in Postfix 3.4 and later.

tlsproxy_tls_ciphers (default: $smtpd_tls_ciphers)
The minimum TLS cipher grade that the Postfix tlsproxy(8) server will
use with opportunistic TLS encryption. See smtpd_tls_ciphers for further
details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dcert_file (default: $smtpd_tls_dcert_file)
File with the Postfix tlsproxy(8) server DSA certificate in PEM format.
This file may also contain the Postfix tlsproxy(8) server private DSA
key. DSA is obsolete and should not be used. See smtpd_tls_dcert_file
for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dh1024_param_file (default: $smtpd_tls_dh1024_param_file)
File with DH parameters that the Postfix tlsproxy(8) server should use
with non-export EDH ciphers. See smtpd_tls_dh1024_param_file for further
details.

This feature is deprecated as of Postfix 3.9. Do not specify.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dh512_param_file (default: $smtpd_tls_dh512_param_file)
File with DH parameters that the Postfix tlsproxy(8) server should use
with export-grade EDH ciphers. See smtpd_tls_dh512_param_file for fur-
ther details. The default SMTP server cipher grade is "medium" with
Postfix releases after the middle of 2015, and as a result export-grade
cipher suites are by default not used.

With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is no
longer supported, and this parameter is silently ignored.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dkey_file (default: $smtpd_tls_dkey_file)
File with the Postfix tlsproxy(8) server DSA private key in PEM format.
This file may be combined with the Postfix tlsproxy(8) server DSA cer-
tificate file specified with $smtpd_tls_dcert_file. DSA is obsolete and
should not be used. See smtpd_tls_dkey_file for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_eccert_file (default: $smtpd_tls_eccert_file)
File with the Postfix tlsproxy(8) server ECDSA certificate in PEM for-
mat. This file may also contain the Postfix tlsproxy(8) server private
ECDSA key. See smtpd_tls_eccert_file for further details. With Postfix
>= 3.4 the preferred way to configure tlsproxy server keys and certifi-
cates is via the "tlsproxy_tls_chain_files" parameter.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_eckey_file (default: $smtpd_tls_eckey_file)
File with the Postfix tlsproxy(8) server ECDSA private key in PEM for-
mat. This file may be combined with the Postfix tlsproxy(8) server
ECDSA certificate file specified with $smtpd_tls_eccert_file. See
smtpd_tls_eckey_file for further details. With Postfix >= 3.4 the pre-
ferred way to configure tlsproxy server keys and certificates is via the
"tlsproxy_tls_chain_files" parameter.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_eecdh_grade (default: $smtpd_tls_eecdh_grade)
The Postfix tlsproxy(8) server security grade for ephemeral ellip-
tic-curve Diffie-Hellman (EECDH) key exchange. See smtpd_tls_eecdh_grade
for further details.

This feature is not used as of Postfix 3.6. Do not specify.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_enable_rpk (default: $smtpd_tls_enable_rpk)
Request that remote SMTP clients send an RFC7250 raw public key instead
of an X.509 certificate, when asking or requiring client authentication.
See $smtpd_tls_enable_rpk for details.

This feature is available in Postfix 3.9 and later.

tlsproxy_tls_exclude_ciphers (default: $smtpd_tls_exclude_ciphers)
List of ciphers or cipher types to exclude from the tlsproxy(8) server
cipher list at all TLS security levels. See smtpd_tls_exclude_ciphers
for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_fingerprint_digest (default: $smtpd_tls_fingerprint_digest)
The message digest algorithm to construct remote SMTP client-certificate
fingerprints. See smtpd_tls_fingerprint_digest for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_key_file (default: $smtpd_tls_key_file)
File with the Postfix tlsproxy(8) server RSA private key in PEM format.
This file may be combined with the Postfix tlsproxy(8) server RSA cer-
tificate file specified with $smtpd_tls_cert_file. See
smtpd_tls_key_file for further details. With Postfix >= 3.4 the pre-
ferred way to configure tlsproxy server keys and certificates is via the
"tlsproxy_tls_chain_files" parameter.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_loglevel (default: $smtpd_tls_loglevel)
Enable additional Postfix tlsproxy(8) server logging of TLS activity.
Each logging level also includes the information that is logged at a
lower logging level. See smtpd_tls_loglevel for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_mandatory_ciphers (default: $smtpd_tls_mandatory_ciphers)
The minimum TLS cipher grade that the Postfix tlsproxy(8) server will
use with mandatory TLS encryption. See smtpd_tls_mandatory_ciphers for
further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_mandatory_exclude_ciphers (default: $smtpd_tls_mandatory_ex-
clude_ciphers)
Additional list of ciphers or cipher types to exclude from the tl-
sproxy(8) server cipher list at mandatory TLS security levels. See
smtpd_tls_mandatory_exclude_ciphers for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_mandatory_protocols (default: $smtpd_tls_mandatory_protocols)
The SSL/TLS protocols accepted by the Postfix tlsproxy(8) server with
mandatory TLS encryption. If the list is empty, the server supports all
available SSL/TLS protocol versions. See smtpd_tls_mandatory_protocols
for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_protocols (default: $smtpd_tls_protocols)
List of TLS protocols that the Postfix tlsproxy(8) server will exclude
or include with opportunistic TLS encryption. See smtpd_tls_protocols
for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_req_ccert (default: $smtpd_tls_req_ccert)
With mandatory TLS encryption, require a trusted remote SMTP client cer-
tificate in order to allow TLS connections to proceed. See
smtpd_tls_req_ccert for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_security_level (default: $smtpd_tls_security_level)
The SMTP TLS security level for the Postfix tlsproxy(8) server; when a
non-empty value is specified, this overrides the obsolete parameters
smtpd_use_tls and smtpd_enforce_tls. See smtpd_tls_security_level for
further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_session_cache_timeout (default: $smtpd_tls_session_cache_timeout)
Obsolete expiration time of Postfix tlsproxy(8) server TLS session cache
information. Since the cache is shared with smtpd(8) and managed by
tlsmgr(8), there is only one expiration time for the SMTP server cache
shared by all three services, namely smtpd_tls_session_cache_timeout.

This feature is available in Postfix 2.8-2.10.

tlsproxy_use_tls (default: $smtpd_use_tls)
Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but
do not require that clients use TLS encryption. See smtpd_use_tls for
further details. Use tlsproxy_tls_security_level instead.

This feature is deprecated as of Postfix 3.9. Specify tlsproxy_tls_secu-
rity_level instead.

This feature is available in Postfix 2.8 and later.

tlsproxy_watchdog_timeout (default: 10s)
How much time a tlsproxy(8) process may take to process local or remote
I/O before it is terminated by a built-in watchdog timer. This is a
safety mechanism that prevents tlsproxy(8) from becoming non-responsive
due to a bug in Postfix itself or in system software. To avoid false
alarms and unnecessary cache corruption this limit cannot be set under
10s.

This feature is available in Postfix 2.8 and later

trace_service_name (default: trace)
The name of the trace service. This service is implemented by the
bounce(8) daemon and maintains a record of mail deliveries and produces
a mail delivery report when verbose delivery is requested with "sendmail
-v".

This feature is available in Postfix 2.1 and later.

transport_delivery_slot_cost (default: $default_delivery_slot_cost)
A transport-specific override for the default_delivery_slot_cost parame-
ter value, where transport is the master.cf name of the message delivery
transport.

Note: transport_delivery_slot_cost parameters will not show up in "post-
conf" command output before Postfix version 2.9. This limitation ap-
plies to many parameters whose name is a combination of a master.cf ser-
vice name and a built-in suffix (in this case: "_delivery_slot_cost").

transport_delivery_slot_discount (default: $default_delivery_slot_discount)
A transport-specific override for the default_delivery_slot_discount pa-
rameter value, where transport is the master.cf name of the message de-
livery transport.

Note: transport_delivery_slot_discount parameters will not show up in
"postconf" command output before Postfix version 2.9. This limitation
applies to many parameters whose name is a combination of a master.cf
service name and a built-in suffix (in this case: "_delivery_slot_dis-
count").

transport_delivery_slot_loan (default: $default_delivery_slot_loan)
A transport-specific override for the default_delivery_slot_loan parame-
ter value, where transport is the master.cf name of the message delivery
transport.

Note: transport_delivery_slot_loan parameters will not show up in "post-
conf" command output before Postfix version 2.9. This limitation ap-
plies to many parameters whose name is a combination of a master.cf ser-
vice name and a built-in suffix (in this case: "_delivery_slot_loan").

transport_destination_concurrency_failed_cohort_limit (default: $default_desti-
nation_concurrency_failed_cohort_limit)
A transport-specific override for the default_destination_concur-
rency_failed_cohort_limit parameter value, where transport is the mas-
ter.cf name of the message delivery transport.

Note: some transport_destination_concurrency_failed_cohort_limit parame-
ters will not show up in "postconf" command output before Postfix ver-
sion 2.9. This limitation applies to many parameters whose name is a
combination of a master.cf service name and a built-in suffix (in this
case: "_destination_concurrency_failed_cohort_limit").

This feature is available in Postfix 2.5 and later.

transport_destination_concurrency_limit (default: $default_destination_concur-
rency_limit)
A transport-specific override for the default_destination_concur-
rency_limit parameter value, where transport is the master.cf name of
the message delivery transport.

Note: some transport_destination_concurrency_limit parameters will not
show up in "postconf" command output before Postfix version 2.9. This
limitation applies to many parameters whose name is a combination of a
master.cf service name and a built-in suffix (in this case: "_destina-
tion_concurrency_limit").

transport_destination_concurrency_negative_feedback (default: $default_destina-
tion_concurrency_negative_feedback)
A transport-specific override for the default_destination_concur-
rency_negative_feedback parameter value, where transport is the mas-
ter.cf name of the message delivery transport.

Note: some transport_destination_concurrency_negative_feedback parame-
ters will not show up in "postconf" command output before Postfix ver-
sion 2.9. This limitation applies to many parameters whose name is a
combination of a master.cf service name and a built-in suffix (in this
case: "_destination_concurrency_negative_feedback").

This feature is available in Postfix 2.5 and later.

transport_destination_concurrency_positive_feedback (default: $default_destina-
tion_concurrency_positive_feedback)
A transport-specific override for the default_destination_concur-
rency_positive_feedback parameter value, where transport is the mas-
ter.cf name of the message delivery transport.

Note: some transport_destination_concurrency_positive_feedback parame-
ters will not show up in "postconf" command output before Postfix ver-
sion 2.9. This limitation applies to many parameters whose name is a
combination of a master.cf service name and a built-in suffix (in this
case: "_destination_concurrency_positive_feedback").

This feature is available in Postfix 2.5 and later.

transport_destination_rate_delay (default: $default_destination_rate_delay)
A transport-specific override for the default_destination_rate_delay pa-
rameter value, where transport is the master.cf name of the message de-
livery transport.

Note: some transport_destination_rate_delay parameters will not show up
in "postconf" command output before Postfix version 2.9. This limita-
tion applies to many parameters whose name is a combination of a mas-
ter.cf service name and a built-in suffix (in this case: "_destina-
tion_rate_delay").

This feature is available in Postfix 2.5 and later.

transport_destination_recipient_limit (default: $default_destination_recipi-
ent_limit)
A transport-specific override for the default_destination_recipi-
ent_limit parameter value, where transport is the master.cf name of the
message delivery transport.

Note: some transport_destination_recipient_limit parameters will not
show up in "postconf" command output before Postfix version 2.9. This
limitation applies to many parameters whose name is a combination of a
master.cf service name and a built-in suffix (in this case: "_destina-
tion_recipient_limit").

transport_extra_recipient_limit (default: $default_extra_recipient_limit)
A transport-specific override for the default_extra_recipient_limit pa-
rameter value, where transport is the master.cf name of the message de-
livery transport.

Note: transport_extra_recipient_limit parameters will not show up in
"postconf" command output before Postfix version 2.9. This limitation
applies to many parameters whose name is a combination of a master.cf
service name and a built-in suffix (in this case: "_extra_recipi-
ent_limit").

transport_initial_destination_concurrency (default: $initial_destination_con-
currency)
A transport-specific override for the initial_destination_concurrency
parameter value, where transport is the master.cf name of the message
delivery transport.

Note: some transport_initial_destination_concurrency parameters will not
show up in "postconf" command output before Postfix version 2.9. This
limitation applies to many parameters whose name is a combination of a
master.cf service name and a built-in suffix (in this case: "_ini-
tial_destination_concurrency").

This feature is available in Postfix 2.5 and later.

transport_maps (default: empty)
Optional lookup tables with mappings from recipient address to (message
delivery transport, next-hop destination). See transport(5) for syntax
details.

This information may override the message delivery transport and/or
next-hop destination that are specified with $local_transport, $vir-
tual_transport, $relay_transport, $default_transport, $sender_depen-
dent_relayhost_maps, $relayhost, $sender_dependent_default_trans-
port_maps, or the recipient domain.

Specify zero or more "type:table" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found. If you use this feature with local files, run "postmap
/etc/postfix/transport" after making a change.

Pattern matching of domain names is controlled by the presence or ab-
sence of "transport_maps" in the parent_domain_matches_subdomains para-
meter value.

For safety reasons, as of Postfix 2.3 this feature does not allow $num-
ber substitutions in regular expression maps.

Examples:

transport_maps = dbm:/etc/postfix/transport
transport_maps = hash:/etc/postfix/transport

transport_minimum_delivery_slots (default: $default_minimum_delivery_slots)
A transport-specific override for the default_minimum_delivery_slots pa-
rameter value, where transport is the master.cf name of the message de-
livery transport.

Note: transport_minimum_delivery_slots parameters will not show up in
"postconf" command output before Postfix version 2.9. This limitation
applies to many parameters whose name is a combination of a master.cf
service name and a built-in suffix (in this case: "_minimum_deliv-
ery_slots").

transport_recipient_limit (default: $default_recipient_limit)
A transport-specific override for the default_recipient_limit parameter
value, where transport is the master.cf name of the message delivery
transport.

Note: some transport_recipient_limit parameters will not show up in
"postconf" command output before Postfix version 2.9. This limitation
applies to many parameters whose name is a combination of a master.cf
service name and a built-in suffix (in this case: "_recipient_limit").

transport_recipient_refill_delay (default: $default_recipient_refill_delay)
A transport-specific override for the default_recipient_refill_delay pa-
rameter value, where transport is the master.cf name of the message de-
livery transport.

Note: transport_recipient_refill_delay parameters will not show up in
"postconf" command output before Postfix version 2.9. This limitation
applies to many parameters whose name is a combination of a master.cf
service name and a built-in suffix (in this case: "_recipient_refill_de-
lay").

This feature is available in Postfix 2.4 and later.

transport_recipient_refill_limit (default: $default_recipient_refill_limit)
A transport-specific override for the default_recipient_refill_limit pa-
rameter value, where transport is the master.cf name of the message de-
livery transport.

Note: transport_recipient_refill_limit parameters will not show up in
"postconf" command output before Postfix version 2.9. This limitation
applies to many parameters whose name is a combination of a master.cf
service name and a built-in suffix (in this case: "_recipient_re-
fill_limit").

This feature is available in Postfix 2.4 and later.

transport_retry_time (default: 60s)
The time between attempts by the Postfix queue manager to contact a mal-
functioning message delivery transport.

transport_time_limit (default: $command_time_limit)
A transport-specific override for the command_time_limit parameter
value, where transport is the master.cf name of the message delivery
transport.

Note: transport_time_limit parameters will not show up in "postconf"
command output before Postfix version 2.9. This limitation applies to
many parameters whose name is a combination of a master.cf service name
and a built-in suffix (in this case: "_time_limit").

transport_transport_rate_delay (default: $default_transport_rate_delay)
A transport-specific override for the default_transport_rate_delay para-
meter value, where the initial transport in the parameter name is the
master.cf name of the message delivery transport.

Note: transport_transport_rate_delay parameters will not show up in
"postconf" command output before Postfix version 2.9. This limitation
applies to many parameters whose name is a combination of a master.cf
service name and a built-in suffix (in this case: "_transport_rate_de-
lay").

trigger_timeout (default: 10s)
The time limit for sending a trigger to a Postfix daemon (for example,
the pickup(8) or qmgr(8) daemon). This time limit prevents programs from
getting stuck when the mail system is under heavy load.

undisclosed_recipients_header (default: see postconf -d output)
Message header that the Postfix cleanup(8) server inserts when a message
contains no To: or Cc: message header. With Postfix 2.8 and later, the
default value is empty. With Postfix 2.4-2.7, specify an empty value to
disable this feature.

Example:

# Default value before Postfix 2.8.
# Note: the ":" and ";" are both required.
undisclosed_recipients_header = To: undisclosed-recipients:;

unknown_address_reject_code (default: 450)
The numerical response code when the Postfix SMTP server rejects a
sender or recipient address because its domain is unknown. This is one
of the possible replies from the restrictions reject_unknown_sender_do-
main and reject_unknown_recipient_domain.

Do not change this unless you have a complete understanding of RFC 5321.

unknown_address_tempfail_action (default: $reject_tempfail_action)
The Postfix SMTP server's action when reject_unknown_sender_domain or
reject_unknown_recipient_domain fail due to a temporary error condition.
Specify "defer" to defer the remote SMTP client request immediately.
With the default "defer_if_permit" action, the Postfix SMTP server con-
tinues to look for opportunities to reject mail, and defers the client
request only if it would otherwise be accepted.

This feature is available in Postfix 2.6 and later.

unknown_client_reject_code (default: 450)
The numerical Postfix SMTP server response code when a client without
valid address <=> name mapping is rejected by the reject_un-
known_client_hostname restriction. The SMTP server always replies with
450 when the mapping failed due to a temporary error condition.

Do not change this unless you have a complete understanding of RFC 5321.

unknown_helo_hostname_tempfail_action (default: $reject_tempfail_action)
The Postfix SMTP server's action when reject_unknown_helo_hostname fails
due to a temporary error condition. Specify "defer" to defer the remote
SMTP client request immediately. With the default "defer_if_permit" ac-
tion, the Postfix SMTP server continues to look for opportunities to re-
ject mail, and defers the client request only if it would otherwise be
accepted.

This feature is available in Postfix 2.6 and later.

unknown_hostname_reject_code (default: 450)
The numerical Postfix SMTP server response code when the hostname speci-
fied with the HELO or EHLO command is rejected by the reject_un-
known_helo_hostname restriction.

Do not change this unless you have a complete understanding of RFC 5321.

unknown_local_recipient_reject_code (default: 550)
The numerical Postfix SMTP server response code when a recipient address
is local, and $local_recipient_maps specifies a list of lookup tables
that does not match the recipient. A recipient address is local when
its domain matches $mydestination, $proxy_interfaces or $inet_inter-
faces.

The default setting is 550 (reject mail) but it is safer to initially
use 450 (try again later) so you have time to find out if your local_re-
cipient_maps settings are OK.

Example:

unknown_local_recipient_reject_code = 450

This feature is available in Postfix 2.0 and later.

unknown_relay_recipient_reject_code (default: 550)
The numerical Postfix SMTP server reply code when a recipient address
matches $relay_domains, and relay_recipient_maps specifies a list of
lookup tables that does not match the recipient address.

This feature is available in Postfix 2.0 and later.

unknown_virtual_alias_reject_code (default: 550)
The Postfix SMTP server reply code when a recipient address matches
$virtual_alias_domains, and $virtual_alias_maps specifies a list of
lookup tables that does not match the recipient address.

This feature is available in Postfix 2.0 and later.

unknown_virtual_mailbox_reject_code (default: 550)
The Postfix SMTP server reply code when a recipient address matches
$virtual_mailbox_domains, and $virtual_mailbox_maps specifies a list of
lookup tables that does not match the recipient address.

This feature is available in Postfix 2.0 and later.

unverified_recipient_defer_code (default: 450)
The numerical Postfix SMTP server response when a recipient address
probe fails due to a temporary error condition.

Unlike elsewhere in Postfix, you can specify 250 in order to accept the
address anyway.

Do not change this unless you have a complete understanding of RFC 5321.

This feature is available in Postfix 2.6 and later.

unverified_recipient_reject_code (default: 450)
The numerical Postfix SMTP server response when a recipient address is
rejected by the reject_unverified_recipient restriction.

Unlike elsewhere in Postfix, you can specify 250 in order to accept the
address anyway.

Do not change this unless you have a complete understanding of RFC 5321.

This feature is available in Postfix 2.1 and later.

unverified_recipient_reject_reason (default: empty)
The Postfix SMTP server's reply when rejecting mail with reject_unveri-
fied_recipient. Do not include the numeric SMTP reply code or the en-
hanced status code. By default, the response includes actual address
verification details.

Example:

unverified_recipient_reject_reason = Recipient address lookup failed

This feature is available in Postfix 2.6 and later.

unverified_recipient_tempfail_action (default: $reject_tempfail_action)
The Postfix SMTP server's action when reject_unverified_recipient fails
due to a temporary error condition. Specify "defer" to defer the remote
SMTP client request immediately. With the default "defer_if_permit" ac-
tion, the Postfix SMTP server continues to look for opportunities to re-
ject mail, and defers the client request only if it would otherwise be
accepted.

This feature is available in Postfix 2.6 and later.

unverified_sender_defer_code (default: 450)
The numerical Postfix SMTP server response code when a sender address
probe fails due to a temporary error condition.

Unlike elsewhere in Postfix, you can specify 250 in order to accept the
address anyway.

Do not change this unless you have a complete understanding of RFC 5321.

This feature is available in Postfix 2.6 and later.

unverified_sender_reject_code (default: 450)
The numerical Postfix SMTP server response code when a recipient address
is rejected by the reject_unverified_sender restriction.

Unlike elsewhere in Postfix, you can specify 250 in order to accept the
address anyway.

Do not change this unless you have a complete understanding of RFC 5321.

This feature is available in Postfix 2.1 and later.

unverified_sender_reject_reason (default: empty)
The Postfix SMTP server's reply when rejecting mail with reject_unveri-
fied_sender. Do not include the numeric SMTP reply code or the enhanced
status code. By default, the response includes actual address verifica-
tion details.

Example:

unverified_sender_reject_reason = Sender address lookup failed

This feature is available in Postfix 2.6 and later.

unverified_sender_tempfail_action (default: $reject_tempfail_action)
The Postfix SMTP server's action when reject_unverified_sender fails due
to a temporary error condition. Specify "defer" to defer the remote SMTP
client request immediately. With the default "defer_if_permit" action,
the Postfix SMTP server continues to look for opportunities to reject
mail, and defers the client request only if it would otherwise be ac-
cepted.

This feature is available in Postfix 2.6 and later.

use_srv_lookup (default: empty)
Enables discovery for the specified service(s) using DNS SRV records.
For example, with "use_srv_lookup = submission" and "relayhost = exam-
ple.com:submission", the Postfix SMTP client will look up DNS SRV
records for _submission._tcp.example.com, and will relay email through
the hosts and ports that are specified with those records. See RFC 2782
for details of the host selection process.

Specify zero or more service names separated by comma and/or whitespace.
Any name in the services(5) database may be specified, though in prac-
tice only submission or submissions (formerly called smtp) make sense.

When SRV record lookup is enabled with use_srv_lookup, you can enclose a
domain name in "[]" to force IP address lookup instead of SRV record
lookup.

Example 1: MUA-to-MTA submission using SRV record lookup for the "sub-
mission" service for domain "example.com". This uses the default SMTP
delivery agent with STARTTLS, and looks up SRV records for "_submis-
sion._tcp.example.com".

/etc/postfix/main.cf:
use_srv_lookup = submission
relayhost = example.com:submission
smtp_tls_security_level = may
...see SASL_README for sasl configuration...

Example 2: MUA-to-MTA submission using SRV record lookup for the "sub-
missions" service for domain "example.org". This uses a dedicated SMTP
delivery agent (smtp-wraptls) with tls_wrappermode turned on, and looks
up SRV records for "_submissions._tcp.example.org".

Note: specify the older name "smtps" instead of "submissions" when a
provider has DNS SRV records like "_smtps._tcp.example.org" instead of
"_submissions._tcp.example.org".

/etc/postfix/main.cf:
use_srv_lookup = submissions
default_transport = smtp-wraptls:example.org:submissions
...see SASL_README for sasl configuration...

/etc/postfix/master.cf:
smtp-wraptls unix ... ... ... ... ... smtp
-o { smtp_tls_wrappermode = yes }
-o { smtp_tls_security_level = encrypt }

Example 3: Sender-dependent selection for a combination of MUA-to-MTA
submission services. This combines examples 1 and 2 with examples of how
to disable SRV and look up IP address records for "smtp-relay.exam-
ple.net" and "smtp-relay.other.example". Again, specify the older name
"smtps" instead of "submissions" when a provider has DNS SRV records
like "_smtps._tcp.example.org" instead of "_submissions._tcp.exam-
ple.org".

/etc/postfix/main.cf:
use_srv_lookup = submission, submissions
sender_dependent_default_transport_maps = inline:{
# Destinations that support SRV record lookup.
{ user1@example.com = smtp:example.com:submission }
{ user2@example.org = smtp-wraptls:example.org:submissions }
# Use [destination] to force IP address lookups.
{ user3@example.net = smtp:[smtp-relay.example.net]:submission }
{ user4@other.example =
smtp-wraptls:[smtp-relay.other.example]:submissions } }
...see SASL_README for sasl configuration...

Example 4: MTA-to-MTA traffic, using SRV record lookup for the SMTP ser-
vice. This is useful for Postfix tests, and may be useful in environ-
ments where ports are dynamically assigned to servers.

/etc/postfix/main.cf:
use_srv_lookup = smtp
# Fall back to MX record lookup when SRV records are unavailable.
#allow_srv_lookup_fallback = yes
#ignore_srv_lookup_error = yes

This feature is available in Postfix 3.8 and later.

verp_delimiter_filter (default: -=+)
The characters Postfix accepts as VERP delimiter characters on the Post-
fix sendmail(1) command line and in SMTP commands.

This feature is available in Postfix 1.1 and later.

virtual_alias_address_length_limit (default: 1000)
The maximal length of an email address after virtual alias expansion.
This stops virtual aliasing loops that increase the address length expo-
nentially.

This feature is available in Postfix 3.0 and later.

virtual_alias_domains (default: $virtual_alias_maps)
Postfix is the final destination for the specified list of virtual alias
domains, that is, domains for which all addresses are aliased to ad-
dresses in other local or remote domains. The SMTP server validates re-
cipient addresses with $virtual_alias_maps and rejects non-existent re-
cipients. See also the virtual alias domain class in the AD-
DRESS_CLASS_README file

This feature is available in Postfix 2.0 and later. The default value is
backwards compatible with Postfix version 1.1.

The default value is $virtual_alias_maps so that you can keep all infor-
mation about virtual alias domains in one place. If you have many
users, it is better to separate information that changes more frequently
(virtual address -> local or remote address mapping) from information
that changes less frequently (the list of virtual domain names).

Specify a list of host or domain names, "/file/name" or "type:table"
patterns, separated by commas and/or whitespace. A "/file/name" pattern
is replaced by its contents; a "type:table" lookup table is matched when
a table entry matches a host or domain name (the lookup result is ig-
nored). Continue long lines by starting the next line with whitespace.
Specify "!pattern" to exclude a host or domain name from the list. The
form "!/file/name" is supported only in Postfix version 2.4 and later.

See also the VIRTUAL_README and ADDRESS_CLASS_README documents for fur-
ther information.

Example:

virtual_alias_domains = virtual1.tld virtual2.tld

virtual_alias_expansion_limit (default: 1000)
The maximal number of addresses that virtual alias expansion produces
from each original recipient.

This feature is available in Postfix 2.1 and later.

virtual_alias_maps (default: $virtual_maps)
Optional lookup tables that are often searched with a full email address
(including domain) and that apply to all recipients: local(8), virtual,
and remote; this is unlike alias_maps that are only searched with an
email address localpart (no domain) and that apply only to local(8) re-
cipients. The virtual_alias_maps table format and lookups are docu-
mented in virtual(5). For an overview of Postfix address manipulations
see the ADDRESS_REWRITING_README document.

This feature is available in Postfix 2.0 and later. The default value is
backwards compatible with Postfix version 1.1.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found. Note: these lookups are recursive.

If you use this feature with indexed files, run "postmap /etc/post-
fix/virtual" after changing the file.

Examples:

virtual_alias_maps = dbm:/etc/postfix/virtual
virtual_alias_maps = hash:/etc/postfix/virtual

virtual_alias_recursion_limit (default: 1000)
The maximal nesting depth of virtual alias expansion. Currently the re-
cursion limit is applied only to the left branch of the expansion graph,
so the depth of the tree can in the worst case reach the sum of the ex-
pansion and recursion limits. This may change in the future.

This feature is available in Postfix 2.1 and later.

virtual_delivery_status_filter (default: $default_delivery_status_filter)
Optional filter for the virtual(8) delivery agent to change the delivery
status code or explanatory text of successful or unsuccessful deliver-
ies. See default_delivery_status_filter for details.

This feature is available in Postfix 3.0 and later.

virtual_destination_concurrency_limit (default: $default_destination_concur-
rency_limit)
The maximal number of parallel deliveries to the same destination via
the virtual message delivery transport. This limit is enforced by the
queue manager. The message delivery transport name is the first field in
the entry in the master.cf file.

virtual_destination_recipient_limit (default: $default_destination_recipi-
ent_limit)
The maximal number of recipients per message for the virtual message de-
livery transport. This limit is enforced by the queue manager. The mes-
sage delivery transport name is the first field in the entry in the mas-
ter.cf file.

Setting this parameter to a value of 1 changes the meaning of vir-
tual_destination_concurrency_limit from concurrency per domain into con-
currency per recipient.

virtual_gid_maps (default: empty)
Lookup tables with the per-recipient group ID for virtual(8) mailbox de-
livery.

This parameter is specific to the virtual(8) delivery agent. It does
not apply when mail is delivered with a different mail delivery program.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

In a lookup table, specify a left-hand side of "@domain.tld" to match
any user in the specified domain that does not have a specific "user@do-
main.tld" entry.

When a recipient address has an optional address extension (user+foo@do-
main.tld), the virtual(8) delivery agent looks up the full address
first, and when the lookup fails, it looks up the unextended address
(user@domain.tld).

Note 1: for security reasons, the virtual(8) delivery agent disallows
regular expression substitution of $1 etc. in regular expression lookup
tables, because that would open a security hole.

Note 2: for security reasons, the virtual(8) delivery agent will
silently ignore requests to use the proxymap(8) server. Instead it will
open the table directly. Before Postfix version 2.2, the virtual(8) de-
livery agent will terminate with a fatal error.

virtual_mailbox_base (default: empty)
A prefix that the virtual(8) delivery agent prepends to all pathname re-
sults from $virtual_mailbox_maps table lookups. This is a safety mea-
sure to ensure that an out of control map doesn't litter the file system
with mailboxes. While virtual_mailbox_base could be set to "/", this
setting isn't recommended.

This parameter is specific to the virtual(8) delivery agent. It does
not apply when mail is delivered with a different mail delivery program.

Example:

virtual_mailbox_base = /var/mail

virtual_mailbox_domains (default: $virtual_mailbox_maps)
Postfix is the final destination for the specified list of domains; mail
is delivered via the $virtual_transport mail delivery transport. By de-
fault this is the Postfix virtual(8) delivery agent. The SMTP server
validates recipient addresses with $virtual_mailbox_maps and rejects
mail for non-existent recipients. See also the virtual mailbox domain
class in the ADDRESS_CLASS_README file.

This parameter expects the same syntax as the mydestination configura-
tion parameter.

This feature is available in Postfix 2.0 and later. The default value is
backwards compatible with Postfix version 1.1.

virtual_mailbox_limit (default: 51200000)
The maximal size in bytes of an individual virtual(8) mailbox or maildir
file, or zero (no limit).

This parameter is specific to the virtual(8) delivery agent. It does
not apply when mail is delivered with a different mail delivery program.

virtual_mailbox_lock (default: see postconf -d output)
How to lock a UNIX-style virtual(8) mailbox before attempting delivery.
For a list of available file locking methods, use the "postconf -l" com-
mand.

This parameter is specific to the virtual(8) delivery agent. It does
not apply when mail is delivered with a different mail delivery program.

This setting is ignored with maildir style delivery, because such deliv-
eries are safe without application-level locks.

Note 1: the dotlock method requires that the recipient UID or GID has
write access to the parent directory of the recipient's mailbox file.

Note 2: the default setting of this parameter is system dependent.

virtual_mailbox_maps (default: empty)
Optional lookup tables with all valid addresses in the domains that
match $virtual_mailbox_domains.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

In a lookup table, specify a left-hand side of "@domain.tld" to match
any user in the specified domain that does not have a specific "user@do-
main.tld" entry.

With the default "virtual_mailbox_domains = $virtual_mailbox_maps",
lookup tables also need entries with a left-hand side of "domain.tld" to
satisfy virtual_mailbox_domain lookups (the right-hand side is required
but will not be used).

The remainder of this text is specific to the virtual(8) delivery agent.
It does not apply when mail is delivered with a different mail delivery
program.

The virtual(8) delivery agent uses this table to look up the per-recipi-
ent mailbox or maildir pathname. If the lookup result ends in a slash
("/"), maildir-style delivery is carried out, otherwise the path is as-
sumed to specify a UNIX-style mailbox file. Note that $virtual_mail-
box_base is unconditionally prepended to this path.

Note 1: for security reasons, the virtual(8) delivery agent disallows
regular expression substitution of $1 etc. in regular expression lookup
tables, because that would open a security hole.

virtual_maps (default: empty)
Optional lookup tables with a) names of domains for which all addresses
are aliased to addresses in other local or remote domains, and b) ad-
dresses that are aliased to addresses in other local or remote domains.
Available before Postfix version 2.0. With Postfix version 2.0 and
later, this is replaced by separate controls: virtual_alias_domains and
virtual_alias_maps.

virtual_minimum_uid (default: 100)
The minimum user ID value that the virtual(8) delivery agent accepts as
a result from $virtual_uid_maps table lookup. Returned values less than
this will be rejected, and the message will be deferred.

This parameter is specific to the virtual(8) delivery agent. It does
not apply when mail is delivered with a different mail delivery program.

virtual_transport (default: virtual)
The default mail delivery transport and next-hop destination for final
delivery to domains listed with $virtual_mailbox_domains. This informa-
tion can be overruled with the transport(5) table.

This feature is available in Postfix 2.0 and later.

virtual_uid_maps (default: empty)
Lookup tables with the per-recipient user ID that the virtual(8) deliv-
ery agent uses while writing to the recipient's mailbox.

This parameter is specific to the virtual(8) delivery agent. It does
not apply when mail is delivered with a different mail delivery program.

Specify zero or more "type:name" lookup tables, separated by whitespace
or comma. Tables will be searched in the specified order until a match
is found.

In a lookup table, specify a left-hand side of "@domain.tld" to match
any user in the specified domain that does not have a specific "user@do-
main.tld" entry.

Note 1: for security reasons, the virtual(8) delivery agent disallows
regular expression substitution of $1 etc. in regular expression lookup
tables, because that would open a security hole.

SEE ALSO
postconf(1), Postfix configuration parameter maintenance
master(5), Postfix daemon configuration maintenance

LICENSE
The Secure Mailer license must be distributed with this software.

AUTHOR(S)
Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA

Wietse Venema
Google, Inc.
111 8th Avenue
New York, NY 10011, USA

Viktor Dukhovni

POSTCONF(5)

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