Anatomy of Postfix
Postfix polls all queues in the directory specified by the queue_directory parameter in your main.cf file. The queue directory is usually /var/spool/postfix. Each queue has its own subdirectory with a name identifying the queue. All messages that Postfix handles stay in these directories until Postfix delivers them. You can determine the status of a message by its queue: incoming, maildrop, deferred, active, hold, or corrupt.
All new messages entering the Postfix queue system get sent to the incoming queue by the cleanup service. New queue files are created with the postfix user as the owner and an access mode of 0600. As soon as a queue file is ready for further processing, the cleanup service changes the queue file mode to 0700 and notifies the queue manager that new mail has arrived. The queue manager ignores incomplete queue files whose mode is 0600.
The queue manager scans the incoming queue when moving new messages into the active queue and makes sure that the active queue resource limits have not been exceeded. By default, the active queue has a maximum of 20,000 messages.
Caution: Once the active queue message limit is reached, the queue manager stops scanning the incoming and deferred queues.
Messages submitted with the sendmail command that have not been sent to the primary Postfix queues by the pickup service await processing in the maildrop queue. You can add messages to the maildrop queue even when Postfix is not running; Postfix will look at them once it is started.
The single-threaded pickup service scans and drains the maildrop queue periodically, as well as upon notification from the postdrop program. The postdrop program is a setgid helper that allows the unprivileged sendmail program to inject mail into the maildrop queue and notify the pickup service of message arrival. (All messages that enter the main Postfix queues do so via the cleanup service.)
If a message still has recipients for which delivery failed for some transient reason, and the message has been delivered to all the recipients possible, Postfix places the message into the deferred queue.
The queue manager scans the deferred queue periodically to put deferred messages back into the active queue. The scan interval is specified with the queue_run_delay configuration parameter. If the deferred and incoming queue scans happen to take place at the same time, the queue manager alternates between the two queues on a per-message basis.
The active queue is somewhat analogous to an operating system's process run queue. Messages in the active queue are ready to be sent, but are not necessarily in the process of being sent.
The queue manager is a delivery agent scheduler that works to ensure fast and fair delivery of mail to all destinations within designated resource limits.
Note: Although most Postfix administrators think of the active queue as a directory on disk, the real active queue is a set of data structures in the memory of the queue manager process.
The administrator can define smtpd access(5) policies and cleanup header and body checks (see Chapter 10) that cause messages to be automatically diverted from normal processing and placed indefinitely in the hold queue. Messages placed in the hold queue stay there until the administrator intervenes. No periodic delivery attempts are made for messages in the hold queue. You can run the postsuper command to manually put messages on hold or to release messages from the hold queue into the deferred queue.
Messages can potentially stay in the hold queue for a time that exceeds the queue file lifetime set by the maximal_queue_lifetime parameter (after which undelivered messages are bounced to the sender). If older messages need to be released from the hold queue, you can use postsuper -r to move them into the maildrop queue, so that the message gets a new timestamp and is given more than one opportunity to be delivered.
Note: The hold queue doesn't play much of a role in Postfix performance; monitoring of the hold queue is typically motivated by tracking spam and malware rather than by performance issues.
The corrupt directory contains damaged queue files. Rather than discarding these, Postfix stores them here so that the (human) postmaster can inspect them using postcat.
Postfix logs a warning about any corrupt files upon startup.
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