Postfix Overview - Introduction - SCN Research

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Postfix Overview - Global Architecture Once an email message is saved to file it stays there until it is read by a mail delivery program. Page 2 of 2 Postfix takes the usual precautions to avoid loss of information: flush and fsync() all data before acknowledging receipt, and check all system call results for error conditions. This style of programming may be new to some people, but I can assure you that it has been standard practice for years in many places. Up one level | Introduction | Goals and features | Global architecture | Queue Management | Security http://www.porcupine.org/postfix-mirror/architecture.html 6/26/01
Postfix Overview - Queue Management Page 1 of 2 Postfix Overview - Queue Management Up one level | Introduction | Goals and features | Global architecture | Queue Management | Security Postfix mail queues Postfix has four different queues: maildrop, incoming, active and deferred (click the upper left-hand icon for the big picture). Locally-posted mail is deposited into the maildrop, and is copied to the incoming queue after some cleaning up. The incoming queue is for mail that is still arriving or that the queue manager hasn't looked at yet. The active queue is a limited-size queue for mail that the queue manager has opened for delivery. Mail that can't be delivered goes to the deferred queue, so that it does not get in the way of other deliveries. The queue manager keeps information in memory about the active queue only. The active queue size is limited on purpose: the queue manager should never run out of working memory because of a peak message workload. Whenever there is space in the active queue, the queue manager lets in one message from the incoming queue and one from the deferred queue. This guarantees that new mail will get through even when there is a large backlog. No thundering herd Implementing a high-performance mail system is one thing. However, no-one would be pleased when Postfix connects to their site and overwhelms it with lots of simultaneous deliveries. This is an issue especially when a site has been down and mail is backed up elsewhere in the network. Postfix tries to be a good network neighbor. When delivering mail to a site, Postfix will initially make no more than two simultaneous connections. As long as deliveries succeed, the concurrency slowly increases up to some configurable limit (or until the host or network is unable to handle the load); concurrency is decreased in the face of trouble. For those familiar with TCP/IP implementation details, Postfix implements its own analog of the TCP slow start algorithm Fairness Apart from the thundering herd controls, the Postfix delivery strategy is based on round-robin selection. The queue manager sorts message recipients in the active queue by destination, and makes round-robin walks along all destination queues. On the average, Postfix will do simultaneous deliveries to the same domain only when there is not enough work to keep all outbound SMTP channels busy. So, when AOL goes off-line and comes back, it should not stop the system from delivering to other sites. When mail arrives faster than Postfix can deliver it, Postfix will favor new mail over delayed mail. The idea is that new mail should be delivered with as little delay as possible; delayed mail can be delivered while the system would otherwise be idle. Exponential backoff Postfix implements per-message exponential backoff. When a message cannot be delivered upon the first http://www.porcupine.org/postfix-mirror/queuing.html 6/26/01
Page 1: Postfix Overview - Introduction Pag
Page 4 and 5: Postfix Overview - Goals and Featur
Page 8 and 9: Postfix Overview - Queue Management
Page 10 and 11: Postfix Overview - Security assista
Page 12 and 13: Postfix Anatomy - Receiving Mail Pa
Page 14 and 15: Postfix Anatomy - Delivering Mail p
Page 16 and 17: Postfix Anatomy - Command-line Util
Page 18 and 19: Postfix Configuration - Basics What
Page 20 and 21: Postfix Configuration - Basics My o
Page 23 and 24: Postfix Configuration - UCE Control
Page 33 and 34: Postfix Rate Controls Page 1 of 4 P
Page 35 and 36: Postfix Rate Controls The defer_tra
Page 37 and 38: Postfix Configuration - Resource Co
Page 39: Postfix Configuration - Resource Co
Page 42 and 43: Postfix Configuration - Address Man
Page 44 and 45: Postfix Configuration - Address Man
Page 47 and 48: ACCESS(5) ACCESS(5) Page 1 of 3 NAM
Page 49: 1 Page 3 of 3 http://www.porcupine.
Page 52 and 53: ited to regular files. For example,
Page 54 and 55: BOUNCE(8) BOUNCE(8) Page 1 of 2 NAM
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CANONICAL(5) CANONICAL(5) Page 1 of
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canonical_maps List of canonical ma
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CONFIGURATION PARAMETERS The follow
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BOUNCE(8) BOUNCE(8) Page 1 of 2 NAM
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FLUSH(8) FLUSH(8) Page 1 of 3 NAME
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fast_flush_purge_time Remove an emp
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Problems and transactions are logge
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parameter. Page 4 of 5 Timeout cont
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LOCAL(8) LOCAL(8) Page 1 of 7 NAME
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estricts delivery to external comma
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details and for default values. Use
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that is written to upon delivery).
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Page 2 of 3 SIGTERM Upon receipt of
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PCRE_TABLE(5) PCRE_TABLE(5) Page 1
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PICKUP(8) PICKUP(8) Page 1 of 2 NAM
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PIPE(8) PIPE(8) Page 1 of 5 NAME pi
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This macro expands to the extension
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ounce(8) non-delivery status report
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-r When updating a table, do not wa
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POSTCAT(1) POSTCAT(1) Page 1 of 1 N
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POSTDROP(1) POSTDROP(1) Page 1 of 2
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POSTFIX(1) POSTFIX(1) Page 1 of 2 N
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POSTKICK(1) POSTKICK(1) Page 1 of 2
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POSTLOCK(1) POSTLOCK(1) Page 1 of 2
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POSTLOG(1) POSTLOG(1) Page 1 of 1 N
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default, postmap creates a new data
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POSTSUPER(1) POSTSUPER(1) Page 1 of
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LICENSE The Secure Mailer license m
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The queue manager implements a vari
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This parameter also limits the size
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REGEXP_TABLE(5) REGEXP_TABLE(5) Pag
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RELOCATED(5) RELOCATED(5) Page 1 of
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Yorktown Heights, NY 10598, USA Pag
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Page 2 of 5 -C config_file (ignored
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Page 4 of 5 DIAGNOSTICS Problems ar
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SHOWQ(8) SHOWQ(8) Page 1 of 1 NAME
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command after a configuration chang
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Page 4 of 4 smtp_helo_timeout Timeo
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Support older Microsoft clients tha
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Page 4 of 5 smtpd_hard_error_limit
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SPAWN(8) SPAWN(8) Page 1 of 2 NAME
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TRANSPORT(5) TRANSPORT(5) Page 1 of
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the entire domain being looked up.
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details and for default values. Use
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VIRTUAL(5) VIRTUAL(5) Page 1 of 4 N
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site is equal to $myorigin, when si
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VIRTUAL(8) VIRTUAL(8) Page 1 of 4 N
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Specifies a minimum uid that will b
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