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 robin
- 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.
RFC 3463 (Enhanced status codes)
RFC 3464 (Delivery status notifications)
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 syslogd(8) or
postlogd(8). 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.
Available before Postfix version 2.5:
- allow_min_user
(no)
- Allow a sender or recipient address to have `-' as the first
character.
Available with Postfix version 2.7 and later:
- default_filter_nexthop
(empty)
- When a content_filter or FILTER request specifies no explicit next-hop
destination, use $default_filter_nexthop instead; when that value is
empty, use the domain in the recipient address.
- minimal_backoff_time
(300s)
- The minimal time between attempts to deliver a deferred message; prior to
Postfix 2.4 the default value was 1000s.
- maximal_backoff_time
(4000s)
- The maximal time between attempts to deliver a deferred message.
- maximal_queue_lifetime
(5d)
- Consider a message as undeliverable, when delivery fails with a temporary
error, and the time in the queue has reached the maximal_queue_lifetime
limit.
- queue_run_delay
(300s)
- The time between deferred queue scans by the queue manager; prior to
Postfix 2.4 the default value was 1000s.
- transport_retry_time
(60s)
- The time between attempts by the Postfix queue manager to contact a
malfunctioning message delivery transport.
Available in Postfix version 2.1 and later:
- bounce_queue_lifetime
(5d)
- Consider a bounce message as undeliverable, when delivery fails with a
temporary error, and the time in the queue has reached the
bounce_queue_lifetime limit.
Available in Postfix version 2.5 and later:
- default_destination_rate_delay
(0s)
- The default amount of delay that is inserted between individual message
deliveries to the same destination and over the same message delivery
transport.
- transport_destination_rate_delay
($default_destination_rate_delay)
- A transport-specific override for the default_destination_rate_delay
parameter value, where transport is the master.cf name of the
message delivery transport.
Available in Postfix version 3.1 and later:
- default_transport_rate_delay
(0s)
- The default amount of delay that is inserted between individual message
deliveries over the same message delivery transport, regardless of
destination.
- transport_transport_rate_delay
($default_transport_rate_delay)
- A transport-specific override for the default_transport_rate_delay
parameter value, where the initial transport in the parameter name
is the master.cf name of the message delivery transport.
/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
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
Preemptive scheduler enhancements:
Patrik Rak
Modra 6
155 00, Prague, Czech Republic
Wietse Venema
Google, Inc.
111 8th Avenue
New York, NY 10011, USA