    NQMGR(8)                                                 NQMGR(8)

    NAME
           nqmgr - Postfix queue manager

    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.

           The nqmgr daemon is responsible for asking  the  bounce(8)
           or defer(8) daemons to send non-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 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  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
                  Minimal delay between warnings that a specific des-
                  tination is clogging up the active queue. Specify 0
                  to disable.

           qmgr_message_active_limit
                  Limit the number of messages in the active queue.

           qmgr_message_recipient_limit
                  Limit the number of in-memory recipients.

                  This  parameter  also limits the size of the short-
                  term, in-memory destination cache.

           qmgr_message_recipient_minimum
                  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
                  to the same destination.

           transport_destination_concurrency_limit
                  Limit  on  the number of parallel deliveries to the
                  same destination, for delivery via the  named  mes-
                  sage transport.

    Recipient controls
           default_destination_recipient_limit
                  Default  limit on the number of recipients per mes-
                  sage transfer.

           transport_destination_recipient_limit
                  Limit on  the  number  of  recipients  per  message
                  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

    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

                                                             NQMGR(8)