    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  addressed to a user listed in the optional relocated
           database is bounced with a "user has  moved  to  new_loca-
           tion" message. See relocated(5) for a precise description.

    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
                  '-'.

           relocated_maps
                  Tables with contact information for users, hosts or
                  domains that no longer exist. See relocated(5).

           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
           relocated(5), format of the "user has moved" table
           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)