NQMGR(8) System Manager's Manual NQMGR(8)
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 dis-
carded. This stops potential loops caused by undeliverable bounce
The nqmgr daemon maintains the following queues:
Inbound mail from the network, or mail picked up by the local
pickup agent 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).
Mail that could not be delivered upon the first attempt. The
queue manager implements exponential backoff by doubling the
time between delivery attempts.
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 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 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.
The nqmgr daemon is responsible for asking the bounce(8) or defer(8)
daemons to send non-delivery reports.
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
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
This strategy eliminates "thundering herd" problems by slowly
adjusting the number of parallel deliveries to the same destina-
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 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-
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 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):
Start a deferred queue scan. If a deferred queue scan is
already in progress, that scan will be restarted as soon as it
Start an incoming queue scan. If an incoming queue scan is
already in progress, that scan will be restarted as soon as it
Ignore deferred queue file time stamps. The request affects the
next deferred queue scan.
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 out-
side world, and it can be run at fixed low privilege in a chrooted
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.
A single queue manager process has to compete for disk access with mul-
tiple front-end processes such as smtpd. A sudden burst of inbound mail
can negatively impact outbound delivery rates.
The following main.cf parameters are especially relevant to this pro-
gram. 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 '-'.
Top-level directory of the Postfix queue.
Active queue controls
In the text below, transport is the first field in a master.cf entry.
Minimal delay between warnings that a specific destination is
clogging up the active queue. Specify 0 to disable.
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
Per message minimum of in-memory recipients.
Default limit on the number of in-memory recipients per trans-
Limit on the number of in-memory recipients, for the named mes-
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
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
Maximal time in days a message is queued before it is sent back
Time in seconds between deferred queue scans. Queue scans do not
Time in seconds between attempts to contact a broken delivery
Initial per-destination concurrency level for parallel delivery
to the same destination.
Default limit on the number of parallel deliveries to the same
Limit on the number of parallel deliveries to the same destina-
tion, 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.
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 incremented by one for each
transport_delivery_slot_cost deliveries handled by transport.
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
Value equal to 0 disables the message preemption for transport.
Message preemption is not attempted at all whenever a message
that can't ever accumulate at least transport_minimum_deliv-
ery_slots available delivery slots is being delivered by trans-
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 deliv-
ery slots required is available, the preemption can happen when
transport_delivery_slot_discount percent of the required amount
plus transport_delivery_slot_loan still remains to be accumu-
lated. Note that the full amount will still have to be accumu-
lated before another preemption can take place later.
Default values for the transport specific parameters described
master(8), process manager
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