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.

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_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)