nqmgr - Postfix queue manager
nqmgr [generic Postfix daemon options]
The nqmgr 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 silently discarded. This stops
potential loops caused by undeliverable bounce notifications.
Mail addressed to a user listed in the optional relocated database is bounced with a "user
has moved to new_location" message. See relocated(5) for a precise description.
The nqmgr daemon maintains the following queues:
Inbound mail from the network, or mail picked up by the local pickup agent from the
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
Mail that could not be delivered upon the first attempt. The queue manager imple-
ments exponential backoff by doubling the time between delivery attempts.
Unreadable or damaged queue files are moved here for inspection.
DELIVERY STATUS REPORTS
The nqmgr 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 main-
tained by the bounce(8) daemon.
defer Per-recipient status information about why mail is delayed. These files are main-
tained by the defer(8) daemon.
The nqmgr daemon is responsible for asking the bounce(8) or defer(8) daemons to send non-
The queue manager implements a variety of strategies for either opening queue files
(input) or for message delivery (output).
This strategy limits the number of messages in the active queue and prevents the
queue manager from running out of memory under heavy load.
When the active queue has room, the queue manager takes one message from the incom-
ing queue and one from the deferred queue. This prevents a large mail backlog from
blocking the delivery of new mail.
This strategy eliminates "thundering herd" problems by slowly adjusting the number
of parallel deliveries to the same destination.
The queue manager sorts delivery requests by destination. Round-robin selection
prevents one destination from dominating deliveries to other destinations.
Mail that cannot be delivered upon the first attempt is deferred. The time inter-
val 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
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):
Start a deferred queue scan. If a deferred queue scan is already in progress, that
scan will be restarted as soon as it finishes.
Start an incoming queue scan. If an incoming queue scan is already in progress,
that scan will be restarted as soon as it finishes.
Ignore deferred queue file time stamps. The request affects the next deferred queue
Purge all information about dead transports and destinations.
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 nqmgr 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 nqmgr daemon does not interact with the outside world.
The nqmgr 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 nqmgr
daemon does not talk to the outside world, and it can be run at fixed low privilege in a
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 pro-
cesses such as smtpd. A sudden burst of inbound mail can negatively impact outbound deliv-
The following main.cf parameters are especially relevant to this program. See the Postfix
main.cf file for syntax details and for default values. Use the postfix reload command
after a configuration change.
Do not bounce recipient addresses that begin with '-'.
Tables with contact information for users, hosts or domains that no longer exist.
Top-level directory of the Postfix queue.
Active queue controls
In the text below, transport is the first field in a master.cf entry.
Limit the number of messages in the active queue.
Limit the number of in-memory recipients.
This parameter also limits the size of the short-term, in-memory destination cache.
Per message minimum of in-memory recipients.
Default limit on the number of in-memory recipients per transport.
Limit on the number of in-memory recipients, for the named message transport.
Default limit on the total number of per transport in-memory recipients that the
preempting messages can have.
Limit on the number of in-memory recipients which all preempting messages delivered
by the transport transport can have.
Minimal time in seconds between delivery attempts of a deferred message.
This parameter also limits the time an unreachable destination is kept in the
short-term, in-memory destination status cache.
Maximal time in seconds between delivery attempts of a deferred message.
Maximal time in days a message is queued before it is sent back as undeliverable.
Time in seconds between deferred queue scans. Queue scans do not overlap.
Time in seconds between attempts to contact a broken delivery transport.
Initial per-destination concurrency level for parallel delivery to the same desti-
Default limit on the number of parallel deliveries to the same destination.
Limit on the number of parallel deliveries to the same destination, for delivery
via the named message transport.
Default limit on the number of recipients per message transfer.
Limit on the number of recipients per message transfer, for the named message
transport_delivery_slot_cost (valid range: 0,2,3...)
This parameter basically controls how often a message delivered by transport can be
preempted by another message. An internal per-message/transport counter is incre-
mented by one for each transport_delivery_slot_cost deliveries handled by trans-
port. This counter represents the number of "available delivery slots" for use by
other messages. Current message can be preempted by another message when that
other message can be delivered using less transport agents than the value of the
"available delivery slots" counter.
Value equal to 0 disables the message preemption for transport.
Message preemption is not attempted at all whenever a message that can't ever accu-
mulate at least transport_minimum_delivery_slots available delivery slots is being
delivered by transport.
transport_delivery_slot_discount (valid range: 0..100)
These parameters speed up the moment when a message preemption can happen. Instead
of waiting until the full amount of delivery slots required is available, the pre-
emption can happen when transport_delivery_slot_discount percent of the required
amount plus transport_delivery_slot_loan still remains to be accumulated. Note that
the full amount will still have to be accumulated before another preemption can
take place later.
Default values for the transport specific parameters described above.
master(8), process manager
relocated(5), format of the "user has moved" table
syslogd(8) system logging
trivial-rewrite(8), address routing
The Secure Mailer license must be distributed with this software.
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA
155 00, Prague, Czech Republic