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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.

       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.

       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.

       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


QMGR(8)                                                   QMGR(8)

              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


QMGR(8)                                                   QMGR(8)

              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.


QMGR(8)                                                   QMGR(8)

       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

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

              Top-level directory of the Postfix queue.

Active queue controls
              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.


QMGR(8)                                                   QMGR(8)

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.

       qmgr_site_hog_factor (valid range: 10..100)
              The  percentage  of  delivery resources that a busy
              mail system will use up for delivery  to  a  single
              site.   With  100%,  mail is delivered in first-in,
              first-out order, so that a burst of  mail  for  one
              site  can  block mail for other destinations.  With
              less than 100%, the excess mail  is  deferred.  The
              deferred  mail  is  delivered in little bursts, the
              remainder of the backlog being deferred again, with
              a lot of I/O activity happening as Postfix searches
              the deferred queue for deliverable mail.

              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.


QMGR(8)                                                   QMGR(8)

       master(8), process manager
       relocated(5), format of the "user has moved" table
       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