qmgr - Postfix queue manager

qmgr [generic Postfix daemon options]

The qmgr(8) daemon awaits the arrival of incoming mail and arranges for its delivery via Postfix delivery processes. The actual mail routing strategy is delegated to the trivial-rewrite(8) daemon. This program expects to be run from the master(8) process manager.

Mail addressed to the local double-bounce address is logged and discarded. This stops potential loops caused by undeliverable bounce notifications.

The qmgr(8) daemon maintains the following queues:

incoming

Inbound mail from the network, or mail picked up by the local pickup(8) daemon from the maildrop directory.

active

Messages that the queue manager has opened for delivery. Only a limited number of messages is allowed to enter the active queue (leaky bucket strategy, for a fixed delivery rate).

deferred

Mail that could not be delivered upon the first attempt. The queue manager implements exponential backoff by doubling the time between delivery attempts.

corrupt

Unreadable or damaged queue files are moved here for inspection.

hold

Messages that are kept "on hold" are kept here until someone sets them free.

The qmgr(8) daemon keeps an eye on per-message delivery status reports in the following directories. Each status report file has the same name as the corresponding message file:

bounce

Per-recipient status information about why mail is bounced. These files are maintained by the bounce(8) daemon.

defer

Per-recipient status information about why mail is delayed. These files are maintained by the defer(8) daemon.

trace

Per-recipient status information as requested with the Postfix "sendmail -v" or "sendmail -bv" command. These files are maintained by the trace(8) daemon.

The qmgr(8) daemon is responsible for asking the bounce(8), defer(8) or trace(8) daemons to send delivery reports.

The queue manager implements a variety of strategies for either opening queue files (input) or for message delivery (output).

leaky bucket

This strategy limits the number of messages in the active queue and prevents the queue manager from running out of memory under heavy load.

fairness

When the active queue has room, the queue manager takes one message from the incoming queue and one from the deferred queue. This prevents a large mail backlog from blocking the delivery of new mail.

slow start

This strategy eliminates "thundering herd" problems by slowly adjusting the number of parallel deliveries to the same destination.

"round The queue manager sorts delivery requests by destination. Round-robin selection prevents one destination from dominating deliveries to other destinations.

exponential backoff

Mail that cannot be delivered upon the first attempt is deferred. The time interval between delivery attempts is doubled after each attempt.

destination status cache

The queue manager avoids unnecessary delivery attempts by maintaining a short-term, in-memory list of unreachable destinations.

preemptive message scheduling

The queue manager attempts to minimize the average per-recipient delay while still preserving the correct per-message delays, using a sophisticated preemptive message scheduling.

On an idle system, the queue manager waits for the arrival of trigger events, or it waits for a timer to go off. A trigger is a one-byte message. Depending on the message received, the queue manager performs one of the following actions (the message is followed by the symbolic constant used internally by the software):

D (QMGR_REQ_SCAN_DEFERRED)

Start a deferred queue scan. If a deferred queue scan is already in progress, that scan will be restarted as soon as it finishes.

I (QMGR_REQ_SCAN_INCOMING)

Start an incoming queue scan. If an incoming queue scan is already in progress, that scan will be restarted as soon as it finishes.

A (QMGR_REQ_SCAN_ALL)

Ignore deferred queue file time stamps. The request affects the next deferred queue scan.

F (QMGR_REQ_FLUSH_DEAD)

Purge all information about dead transports and destinations.

W (TRIGGER_REQ_WAKEUP)

Wakeup call, This is used by the master server to instantiate servers that should not go away forever. The action is to start an incoming queue scan.

The qmgr(8) daemon reads an entire buffer worth of triggers. Multiple identical trigger requests are collapsed into one, and trigger requests are sorted so that A and F precede D and I. Thus, in order to force a deferred queue run, one would request A F D; in order to notify the queue manager of the arrival of new mail one would request I.

None. The qmgr(8) daemon does not interact with the outside world.

The qmgr(8) daemon is not security sensitive. It reads single-character messages from untrusted local users, and thus may be susceptible to denial of service attacks. The qmgr(8) daemon does not talk to the outside world, and it can be run at fixed low privilege in a chrooted environment.

Problems and transactions are logged to the syslog daemon. Corrupted message files are saved to the corrupt queue for further inspection.

Depending on the setting of the notify_classes parameter, the postmaster is notified of bounces and of other trouble.

A single queue manager process has to compete for disk access with multiple front-end processes such as cleanup(8). A sudden burst of inbound mail can negatively impact outbound delivery rates.

Changes to main.cf are not picked up automatically as qmgr(8) is a persistent process. Use the "postfix reload" command after a configuration change.

The text below provides only a parameter summary. See postconf(5) for more details including examples.

In the text below, transport is the first field in a master.cf entry.

allow_min_user (no)

Allow a recipient address to have `-' as the first character.

qmgr_clog_warn_time (300s)

The minimal delay between warnings that a specific destination is clogging up the Postfix active queue.

qmgr_message_active_limit (20000)

The maximal number of messages in the active queue.

qmgr_message_recipient_limit (20000)

The maximal number of recipients held in memory by the Postfix queue manager, and the maximal size of the size of the short-term, in-memory "dead" destination status cache.

qmgr_message_recipient_minimum (10)

The minimal number of in-memory recipients for any message.

default_recipient_limit (10000)

The default per-transport upper limit on the number of in-memory recipients.

transport_recipient_limit ($default_recipient_limit)

Idem, for delivery via the named message transport.

default_extra_recipient_limit (1000)

The default value for the extra per-transport limit imposed on the number of in-memory recipients.

transport_extra_recipient_limit ($default_extra_recipient_limit)

Idem, for delivery via the named message transport.

initial_destination_concurrency (5)

The initial per-destination concurrency level for parallel delivery to the same destination.

default_destination_concurrency_limit (20)

The default maximal number of parallel deliveries to the same destination.

transport_destination_concurrency_limit ($default_destination_concurrency_limit)

Idem, for delivery via the named message transport.

default_destination_recipient_limit (50)

The default maximal number of recipients per message delivery.

transport_destination_recipient_limit ($default_destination_recipient_limit)

Idem, for delivery via the named message transport.

default_delivery_slot_cost (5)

How often the Postfix queue manager's scheduler is allowed to preempt delivery of one message with another.

transport_delivery_slot_cost ($default_delivery_slot_cost)

Idem, for delivery via the named message transport.

default_minimum_delivery_slots (3)

How many recipients a message must have in order to invoke the Postfix queue manager's scheduling algorithm at all.

transport_minimum_delivery_slots ($default_minimum_delivery_slots)

Idem, for delivery via the named message transport.

default_delivery_slot_discount (50)

The default value for transport-specific _delivery_slot_discount settings.

transport_delivery_slot_discount ($default_delivery_slot_discount)

Idem, for delivery via the named message transport.

default_delivery_slot_loan (3)

The default value for transport-specific _delivery_slot_loan settings.

transport_delivery_slot_loan ($default_delivery_slot_loan)

Idem, for delivery via the named message transport.

minimal_backoff_time (1000s)

The minimal time between attempts to deliver a deferred message.

maximal_backoff_time (4000s)

The maximal time between attempts to deliver a deferred message.

maximal_queue_lifetime (5d)

The maximal time a message is queued before it is sent back as undeliverable.

queue_run_delay (1000s)

The time between deferred queue scans by the queue manager.

transport_retry_time (60s)

The time between attempts by the Postfix queue manager to contact a malfunctioning message delivery transport.

Available in Postfix version 2.1 and later:

bounce_queue_lifetime (5d)

The maximal time a bounce message is queued before it is considered undeliverable.

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.

defer_transports (empty)

The names of message delivery transports that should not be delivered to unless someone issues "sendmail -q" or equivalent.

helpful_warnings (yes)

Log warnings about problematic configuration settings, and provide helpful suggestions.

ipc_timeout (3600s)

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

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 (postfix)

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/incoming, incoming queue
/var/spool/postfix/active, active queue
/var/spool/postfix/deferred, deferred queue
/var/spool/postfix/bounce, non-delivery status
/var/spool/postfix/defer, non-delivery status
/var/spool/postfix/trace, delivery status

trivial-rewrite(8), address routing
bounce(8), delivery status reports
postconf(5), configuration parameters
master(5), generic daemon options
master(8), process manager
syslogd(8), system logging

Use "postconf readme_directory" or "postconf html_directory" to locate this information.

SCHEDULER_README, scheduling algorithm
QSHAPE_README, Postfix queue analysis

The Secure Mailer license must be distributed with this software.

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

Scheduler enhancements: Patrik Rak Modra 6 155 00, Prague, Czech Republic