QMGR(8)                                                   QMGR(8)
       qmgr - Postfix queue manager
       qmgr [generic Postfix daemon options]
       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.
       The qmgr daemon maintains the following queues:
              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).
              Mail that could not be  delivered  upon  the  first
              attempt.  The  queue manager implements exponential
              backoff  by  doubling  the  time  between  delivery
              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.
       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
       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.
       The  queue  manager implements a variety of strategies for
       either opening queue files (input) or for message delivery
       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.
              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-
       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.
       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):
              Start a deferred queue scan.  If a  deferred  queue
              scan  is  already  in  progress,  that scan will be
              restarted as soon as it finishes.
              Start an incoming queue scan. If an incoming  queue
              scan  is  already  in  progress,  that scan will be
              restarted as soon as it finishes.
              Ignore deferred queue file time stamps. The request
              affects the next deferred queue scan.
              Purge  all  information  about  dead transports and
              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
       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.
       None. The qmgr daemon does not interact with  the  outside
       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.
       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-
       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.
       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.
              Do  not  bounce recipient addresses that begin with
              Top-level directory of the Postfix queue.
Active queue controls
              Minimal delay between warnings that a specific des-
              tination is clogging up the active queue. Specify 0
              to disable.
              Limit the number of messages in the active queue.
              Limit the number of in-memory recipients.
              This parameter also limits the size of  the  short-
              term, in-memory destination cache.
Timing controls
              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 time in seconds between  delivery  attempts
              of a deferred message.
              Maximal  time in days a message is queued before it
              is sent back as undeliverable.
              Time in seconds between deferred queue scans. Queue
              scans do not overlap.
              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 per-destination concurrency level for  par-
              allel delivery to the same destination.
              Default  limit on the number of parallel deliveries
              to the same destination.
              Limit on the number of parallel deliveries  to  the
              same  destination,  for delivery via the named mes-
              sage transport.
Recipient controls
              Default limit on the number of recipients per  mes-
              sage transfer.
              Limit  on  the  number  of  recipients  per message
              transfer, for the named message transport.
       master(8), process manager
       syslogd(8) system logging
       trivial-rewrite(8), address routing
       The Secure Mailer license must be  distributed  with  this
       Wietse Venema
       IBM T.J. Watson Research
       P.O. Box 704
       Yorktown Heights, NY 10598, USA