QMGR(8) QMGR(8)
NAME
qmgr - Postfix queue manager
SYNOPSIS
qmgr [generic Postfix daemon options]
DESCRIPTION
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 triv-
ial-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.
MAIL QUEUES
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 direc-
tory.
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.
DELIVERY STATUS REPORTS
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" com-
mand. 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.
STRATEGIES
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 deliver-
ies to the same destination.
round robin
The queue manager sorts delivery requests by desti-
nation. Round-robin selection prevents one desti-
nation from dominating deliveries to other destina-
tions.
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 cor-
rect per-message delays, using a sophisticated pre-
emptive message scheduling.
TRIGGERS
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 for-
ever. The action is to start an incoming queue
scan.
The qmgr(8) daemon reads an entire buffer worth of trig-
gers. 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.
STANDARDS
RFC 3463 (Enhanced status codes)
RFC 3464 (Delivery status notifications)
SECURITY
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 environ-
ment.
DIAGNOSTICS
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 trou-
ble.
BUGS
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.
CONFIGURATION PARAMETERS
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 mas-
ter.cf entry.
COMPATIBILITY CONTROLS
allow_min_user (no)
Allow a recipient address to have `-' as the first
character.
ACTIVE QUEUE CONTROLS
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" desti-
nation status cache.
qmgr_message_recipient_minimum (10)
The minimal number of in-memory recipients for any
message.
default_recipient_limit (20000)
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_recipi-
ent_limit)
Idem, for delivery via the named message transport.
Available in Postfix version 2.4 and later:
default_recipient_refill_limit (100)
The default per-transport limit on the number of
recipients refilled at once.
transport_recipient_refill_limit ($default_recipi-
ent_refill_limit)
Idem, for delivery via the named message transport.
default_recipient_refill_delay (5s)
The default per-transport maximum delay between
recipients refills.
transport_recipient_refill_delay ($default_recipi-
ent_refill_delay)
Idem, for delivery via the named message transport.
DELIVERY CONCURRENCY CONTROLS
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 ($default_destina-
tion_concurrency_limit)
Idem, for delivery via the named message transport.
RECIPIENT SCHEDULING CONTROLS
default_destination_recipient_limit (50)
The default maximal number of recipients per mes-
sage delivery.
transport_destination_recipient_limit ($default_destina-
tion_recipient_limit)
Idem, for delivery via the named message transport.
MESSAGE SCHEDULING CONTROLS
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 ($default_delivery_slot_cost)
Idem, for delivery via the named message transport.
default_minimum_delivery_slots (3)
How many recipients a message must have in order to
invoke the Postfix queue manager's scheduling algo-
rithm at all.
transport_minimum_delivery_slots ($default_minimum_deliv-
ery_slots)
Idem, for delivery via the named message transport.
default_delivery_slot_discount (50)
The default value for transport-specific _deliv-
ery_slot_discount settings.
transport_delivery_slot_discount ($default_deliv-
ery_slot_discount)
Idem, for delivery via the named message transport.
default_delivery_slot_loan (3)
The default value for transport-specific _deliv-
ery_slot_loan settings.
transport_delivery_slot_loan ($default_delivery_slot_loan)
Idem, for delivery via the named message transport.
OTHER RESOURCE AND RATE CONTROLS
minimal_backoff_time (version dependent)
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 (version dependent)
The time between deferred queue scans by the queue
manager.
transport_retry_time (60s)
The time between attempts by the Postfix queue man-
ager 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.
MISCELLANEOUS CONTROLS
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 deliver mail unless someone issues
"sendmail -q" or equivalent.
delay_logging_resolution_limit (2)
The maximal number of digits after the decimal
point when logging sub-second delay values.
helpful_warnings (yes)
Log warnings about problematic configuration set-
tings, 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 direc-
tory.
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".
FILES
/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
SEE ALSO
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
README FILES
SCHEDULER_README, scheduling algorithm
QSHAPE_README, Postfix queue analysis
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
Scheduler enhancements:
Patrik Rak
Modra 6
155 00, Prague, Czech Republic
QMGR(8)