The incoming message queue holds the new mail that has been received. Each message is identified with a unique file name. Messages are moved to the active queue when there is room. If there are no problems, message move through this queue very quickly.
The active message queue holds messages that are ready to be sent. The MTA sets a limit to the number of messages that can be in the active queue at any one time. From here, messages are moved to and from the anti-virus and anti-spam filters before being delivered to another queue.
Messages that cannot be delivered are placed in the deferred queue. The reasons for the delivery failures are documented in a file in the deferred queue. This queue is scanned frequently to resend the message. If the message cannot be sent after the set number of delivery attempts, the message fails. The message is bounced back to the original sender. The default for the bounce queue lifetime is five days.
The held message queue keeps mail that could not be processed. Messages stay in this queue until the administrator moves them. No periodic delivery attempts are made for messages in the held queue.
The
corrupt queue stores damaged unreadable messages.
