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QMGR(8)                     System Manager's Manual                    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 trivial-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  dis-
       carded.   This  stops  potential  loops  caused by undeliverable bounce

       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 attempts.

              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 status 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.

       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.

              When the active queue has room, the queue manager takes one mes-
              sage  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 deliveries to the same destina-

       round robin
              The  queue  manager  sorts  delivery  requests  by  destination.
              Round-robin selection prevents one destination  from  dominating
              deliveries to other destinations.

       exponential backoff
              Mail  that  cannot  be  delivered  upon  the  first  attempt  is
              deferred.  The time interval between delivery attempts  is  dou-
              bled after each attempt.

       destination status cache
              The  queue manager avoids unnecessary delivery attempts by main-
              taining a short-term, in-memory  list  of  unreachable  destina-

       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 mes-
       sage.   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

              Start  an  incoming  queue  scan.  If  an incoming queue scan is
              already in progress, that scan will be restarted as soon  as  it

              Ignore  deferred queue file time stamps. The request affects the
              next deferred queue scan.

              Purge all information about dead transports and destinations.

              Wakeup call, This is used by the master  server  to  instantiate
              servers  that should not go away forever. 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 precede 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 world.

       The qmgr daemon is not security sensitive.  It  reads  single-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 environ-

       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 postmas-
       ter is notified of bounces and of other trouble.

       A single queue manager process has to compete for disk access with mul-
       tiple front-end processes such as smtpd. A sudden burst of inbound mail
       can negatively impact outbound delivery rates.

       The following main.cf parameters are especially relevant to  this  pro-
       gram.  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  destination  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

              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

              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

              Time  in  seconds  between attempts to contact a broken delivery

Concurrency controls
       In the text below, transport is the first field in a master.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 performance 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 beginning of a
              large list receive a burst of messages immediately, while recip-
              ients  near the end of that list receive that same burst of mes-
              sages a whole day later.

              Initial per-destination concurrency level for parallel  delivery
              to the same destination.

              Default  limit  on the number of parallel deliveries to the same

              Limit on the number of parallel deliveries to the same  destina-
              tion, for delivery via the named message transport.

Recipient controls
              Default limit on the number of recipients per message 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 software.

       Wietse Venema
       IBM T.J. Watson Research
       P.O. Box 704
       Yorktown Heights, NY 10598, USA