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 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 dis-
carded. 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 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.
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" command. These files are main-
tained 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 mes-
sage 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 destina-
tion.
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 dou-
bled after each attempt.
destination status cache
The queue manager avoids unnecessary delivery attempts by main-
taining a short-term, in-memory list of unreachable destina-
tions.
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.
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 mes-
sage. 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.
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 out-
side world, and it can be run at fixed low privilege in a chrooted
environment.
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 postmas-
ter is notified of bounces and of other trouble.
BUGS
A single queue manager process has to compete for disk access with mul-
tiple 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 per-
sistent 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.
COMPATIBILITY CONTROLS
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 $default_filter_nexthop instead; when
that value is empty, use the domain in the recipient address.
ACTIVE QUEUE CONTROLS
qmgr_clog_warn_time (300s)
The minimal delay between warnings that a specific destination
is clogging up the Postfix active queue.
qmgr_message_active_limit (20000)
The maximal number of messages in the active queue.
qmgr_message_recipient_limit (20000)
The maximal number of recipients held in memory by the Postfix
queue manager, and the maximal size of the short-term, in-memory
"dead" destination status cache.
qmgr_message_recipient_minimum (10)
The minimal number of in-memory recipients for any message.
default_recipient_limit (20000)
The default per-transport upper limit on the number of in-memory
recipients.
transport_recipient_limit ($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 ($default_extra_recipient_limit)
Idem, for delivery via the named message transport.
Available in Postfix version 2.4 and later:
default_recipient_refill_limit (100)
The default per-transport limit on the number of recipients
refilled at once.
transport_recipient_refill_limit ($default_recipient_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 ($default_recipient_refill_delay)
Idem, for delivery via the named message transport.
DELIVERY CONCURRENCY CONTROLS
initial_destination_concurrency (5)
The initial per-destination concurrency level for parallel
delivery to the same destination.
default_destination_concurrency_limit (20)
The default maximal number of parallel deliveries to the same
destination.
transport_destination_concurrency_limit ($default_destination_concur-
rency_limit)
Idem, for delivery via the named message transport.
Available in Postfix version 2.5 and later:
transport_initial_destination_concurrency ($initial_destination_concur-
rency)
Initial concurrency for delivery via the named message trans-
port.
default_destination_concurrency_failed_cohort_limit (1)
How many pseudo-cohorts must suffer connection or handshake
failure before a specific destination is considered unavailable
(and further delivery is suspended).
transport_destination_concurrency_failed_cohort_limit ($default_desti-
nation_concurrency_failed_cohort_limit)
Idem, for delivery via the named message transport.
default_destination_concurrency_negative_feedback (1)
The per-destination amount of delivery concurrency negative
feedback, after a delivery completes with a connection or hand-
shake failure.
transport_destination_concurrency_negative_feedback ($default_destina-
tion_concurrency_negative_feedback)
Idem, for delivery via the named message transport.
default_destination_concurrency_positive_feedback (1)
The per-destination amount of delivery concurrency positive
feedback, after a delivery completes without connection or hand-
shake failure.
transport_destination_concurrency_positive_feedback ($default_destina-
tion_concurrency_positive_feedback)
Idem, for delivery via the named message transport.
destination_concurrency_feedback_debug (no)
Make the queue manager's feedback algorithm verbose for perfor-
mance analysis purposes.
RECIPIENT SCHEDULING CONTROLS
default_destination_recipient_limit (50)
The default maximal number of recipients per message delivery.
transport_destination_recipient_limit ($default_destination_recipi-
ent_limit)
Idem, for delivery via the named message transport.
MESSAGE SCHEDULING CONTROLS
default_delivery_slot_cost (5)
How often the Postfix queue manager's scheduler is allowed to
preempt delivery of one message with another.
transport_delivery_slot_cost ($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 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.
OTHER RESOURCE AND RATE CONTROLS
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 maxi-
mal_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 con-
tact 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
deliveries to the same destination; the resulting behavior
depends on the value of the corresponding per-destination recip-
ient limit.
transport_destination_rate_delay $default_destination_rate_delay
Idem, for delivery via the named message transport.
Available in Postfix version 3.1 and later:
default_transport_rate_delay (0s)
The default amount of delay that is inserted between individual
deliveries over the same message delivery transport, regardless
of destination.
transport_transport_rate_delay $default_transport_rate_delay
Idem, for delivery via the named message transport.
SAFETY CONTROLS
qmgr_daemon_timeout (1000s)
How much time a Postfix queue manager process may take to handle
a request before it is terminated by a built-in watchdog timer.
qmgr_ipc_timeout (60s)
The time limit for the queue manager to send or receive informa-
tion over an internal communication channel.
Available in Postfix version 3.1 and later:
address_verify_pending_request_limit (see 'postconf -d' output)
A safety limit that prevents address verification requests from
overwhelming the Postfix queue.
MISCELLANEOUS CONTROLS
config_directory (see 'postconf -d' output)
The default location of the Postfix main.cf and master.cf con-
figuration files.
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 log-
ging sub-second delay values.
helpful_warnings (yes)
Log warnings about problematic configuration settings, and pro-
vide helpful suggestions.
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)
A prefix that is prepended to the process name in syslog
records, so that, for example, "smtpd" becomes "prefix/smtpd".
Available in Postfix version 3.0 and later:
confirm_delay_cleared (no)
After sending a "your message is delayed" notification, inform
the sender when the delay clears up.
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
Preemptive scheduler enhancements:
Patrik Rak
Modra 6
155 00, Prague, Czech Republic
Wietse Venema
Google, Inc.
111 8th Avenue
New York, NY 10011, USA
QMGR(8)