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
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
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.
Postfix implements per-message exponential backoff. When a message
cannot be delivered upon the first attempt, the queue manager gives
the queue file a time stamp that is offset into the future by some
configurable amount of time. Queue files with future time stamps
are normally ignored by the queue manager.
Whenever a repeat delivery attempt fails, the queue file time stamp
is moved into the future by an amount of time equal to the age of
the message. Thus, the time between delivery attempts doubles each
time. This strategy effectively implements exponential backoff.