flush − Postfix fast flush server


flush [generic Postfix daemon options]


The flush(8) server maintains a record of deferred mail by destination. This information is used to improve the performance of the SMTP ETRN request, and of its command-line equivalent, "sendmail -qR" or "postqueue -f". This program expects to be run from the master(8) process manager.

The record is implemented as a per-destination logfile with as contents the queue IDs of deferred mail. A logfile is append-only, and is truncated when delivery is requested for the corresponding destination. A destination is the part on the right-hand side of the right-most @ in an email address.

Per-destination logfiles of deferred mail are maintained only for eligible destinations. The list of eligible destinations is specified with the fast_flush_domains configuration parameter, which defaults to $relay_domains.

This server implements the following requests:
sitename queueid

Inform the flush(8) server that the message with the specified queue ID is queued for the specified destination.

send_site sitename

Request delivery of mail that is queued for the specified destination.

send_file queueid

Request delivery of the specified deferred message.


Refresh non-empty per-destination logfiles that were not read in $fast_flush_refresh_time hours, by simulating send requests (see above) for the corresponding destinations.

Delete empty per-destination logfiles that were not updated in $fast_flush_purge_time days.

This request completes in the background.


Do a refresh for all per-destination logfiles.


The flush(8) server is not security-sensitive. It does not talk to the network, and it does not talk to local users. The fast flush server can run chrooted at fixed low privilege.


Problems and transactions are logged to syslogd(8).


Fast flush logfiles are truncated only after a "send" request, not when mail is actually delivered, and therefore can accumulate outdated or redundant data. In order to maintain sanity, "refresh" must be executed periodically. This can be automated with a suitable wakeup timer setting in the master.cf configuration file.

Upon receipt of a request to deliver mail for an eligible destination, the flush(8) server requests delivery of all messages that are listed in that destination’s logfile, regardless of the recipients of those messages. This is not an issue for mail that is sent to a relay_domains destination because such mail typically only has recipients in one domain.


Changes to main.cf are picked up automatically as flush(8) processes run for only a limited amount of time. Use the command "postfix reload" to speed up a change.

The text below provides only a parameter summary. See postconf(5) for more details including examples.
config_directory (see ’postconf -d’ output)

The default location of the Postfix main.cf and master.cf configuration files.

daemon_timeout (18000s)

How much time a Postfix daemon process may take to handle a request before it is terminated by a built-in watchdog timer.

fast_flush_domains ($relay_domains)

Optional list of destinations that are eligible for per-destination logfiles with mail that is queued to those destinations.

fast_flush_refresh_time (12h)

The time after which a non-empty but unread per-destination "fast flush" logfile needs to be refreshed.

fast_flush_purge_time (7d)

The time after which an empty per-destination "fast flush" logfile is deleted.

ipc_timeout (3600s)

The time limit for sending or receiving information over an internal communication channel.

max_idle (100s)

The maximum amount of time that an idle Postfix daemon process waits for an incoming connection before terminating voluntarily.

max_use (100)

The maximal number of incoming connections that a Postfix daemon process will service before terminating voluntarily.

parent_domain_matches_subdomains (see ’postconf -d’ output)

What Postfix features match subdomains of "domain.tld" automatically, instead of requiring an explicit ".domain.tld" pattern.

process_id (read-only)

The process ID of a Postfix command or daemon process.

process_name (read-only)

The process name of a Postfix command or daemon process.

queue_directory (see ’postconf -d’ output)

The location of the Postfix top-level queue directory.

syslog_facility (mail)

The syslog facility of Postfix logging.

syslog_name (see ’postconf -d’ output)

The mail system name that is prepended to the process name in syslog records, so that "smtpd" becomes, for example, "postfix/smtpd".


/var/spool/postfix/flush, "fast flush" logfiles.


smtpd(8), SMTP server
qmgr(8), queue manager
postconf(1), configuration parameters
master(5), generic daemon options
master(5), process manager
syslogd(8), system logging


Use "postconf readme_directory" or "postconf html_directory" to locate this information.
ETRN_README, Postfix ETRN howto


The Secure Mailer license must be distributed with this software.


This service was introduced with Postfix version 1.0.


Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA


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