    QMGR(8)                                                   QMGR(8)

    NAME
           qmgr - Postfix queue manager

    SYNOPSIS
           qmgr [generic Postfix daemon options]

    DESCRIPTION
           The  qmgr  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 qmgr 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 qmgr 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 qmgr 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.

    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 qmgr 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  qmgr daemon does not interact with the outside
           world.

    SECURITY
           The qmgr 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
           qmgr daemon does not talk to the outside world, and it can
           be run at fixed low privilege in a chrooted environment.

    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
           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.

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

           qmgr_fudge_factor (valid range: 10..100)
                  The  percentage  of  delivery resources that a busy
                  mail system will use up for  delivery  of  a  large
                  mailing  list  message.  With 100%, delivery of one
                  message does not begin before the previous  message
                  has  been  delivered.  This results in good perfor-
                  mance for large mailing lists, but results in  poor
                  response  time for one-to-one mail.  With less than
                  100%, response time for one-to-one  mail  improves,
                  but  large  mailing  list delivery performance suf-
                  fers. In the worst case, recipients near the begin-
                  ning  of  a  large list receive a burst of messages
                  immediately, while recipients near the end of  that
                  list  receive  that  same burst of messages a whole
                  day later.

           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.

    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

                                                              QMGR(8)