Note: this web page is no longer maintained. It exists only to avoid breaking links in web pages that describe earlier versions of the Postfix mail system.Up one level | Introduction | Goals and features | Global architecture | Queue Management | Security
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.
In addition to the queues mentioned above Postfix also maintains two parking spaces. The hold queue is for mail that is frozen in the queue; no delivery attempts are made until someone releases these messages with the postsuper command. The corrupt directory is for damaged queue files. Rather than discarding these, Postfix leaves them here for human inspection.
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
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.
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.