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nqmgr



SYNOPSIS

       nqmgr [generic Postfix daemon options]


DESCRIPTION

       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 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
       silently discarded.  This stops potential loops caused  by
       undeliverable bounce notifications.


MAIL QUEUES

       The nqmgr daemon maintains the following queues:

       incoming
              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).

       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 nqmgr daemon keeps an eye on per-message delivery sta­
       tus 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.

       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)
       Multiple  identical  trigger  requests  are collapsed into
       one, and trigger requests are sorted so that A and F  pre­
       cede  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

       None.  The nqmgr daemon does not interact with the outside
       world.


SECURITY

       The nqmgr daemon is not security sensitive. It reads  sin­
       gle-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  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 smtpd.  A
       sudden  burst  of  inbound mail can negatively impact out­
       bound delivery rates.


CONFIGURATION PARAMETERS

       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.


Miscellaneous

       allow_min_user
              Do  not  bounce recipient addresses that begin with
              '-'.

       queue_directory
              Top-level directory of the Postfix queue.


Active queue controls

       In the text below, transport is the first field in a  mas­
       ter.cf entry.

       qmgr_clog_warn_time
              Per message minimum of in-memory recipients.

       default_recipient_limit
              Default limit on the number of in-memory recipients
              per transport.

       transport_recipient_limit
              Limit  on  the  number of in-memory recipients, for
              the named message transport.

       default_extra_recipient_limit
              Default limit on the total number of per  transport
              in-memory  recipients  that the preempting messages
              can have.

       transport_extra_recipient_limit
              Limit on the number of in-memory  recipients  which
              all  preempting messages delivered by the transport
              transport can have.


Timing controls

       minimal_backoff_time
              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_backoff_time
              Maximal  time  in seconds between delivery attempts
              of a deferred message.

       maximal_queue_lifetime
              Maximal time in days a message is queued before  it
              is sent back as undeliverable.

       queue_run_delay
              Time in seconds between deferred queue scans. Queue
              scans do not overlap.

       transport_retry_time
              Time in seconds between attempts to contact a  bro­
              ken delivery transport.


Concurrency controls

       initial_destination_concurrency
              Initial  per-destination concurrency level for par­
              allel delivery to the same destination.

       default_destination_concurrency_limit
              Default limit on the number of parallel  deliveries
              transfer, for the named message transport.


Message scheduling

       transport_delivery_slot_cost (valid range: 0,2,3...)
              This  parameter basically controls how often a mes­
              sage delivered by transport  can  be  preempted  by
              another message.  An internal per-message/transport
              counter is  incremented  by  one  for  each  trans­
              port_delivery_slot_cost   deliveries   handled   by
              transport. This counter represents  the  number  of
              "available  delivery  slots"  for use by other mes­
              sages.  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.

       transport_minimum_delivery_slots
              Message preemption is not attempted at all whenever
              a  message  that  can't  ever  accumulate  at least
              transport_minimum_delivery_slots available delivery
              slots is being delivered by transport.

       transport_delivery_slot_discount (valid range: 0..100)

       transport_delivery_slot_loan
              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 preemption can  happen  when  trans­
              port_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_delivery_slot_cost

       default_minimum_delivery_slots

       default_delivery_slot_discount

       default_delivery_slot_loan
              Default  values  for the transport specific parame­
              ters described above.


SEE ALSO

       master(8), process manager
       syslogd(8) system logging
       trivial-rewrite(8), address routing
       155 00, Prague, Czech Republic

                                                         NQMGR(8)
  
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