    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 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
                  '-'.

           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
           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)