When the Zimbra MTA receives mail, it routes the mail through a series of queues to manage delivery. The Zimbra MTA maintains four queues where mail is temporarily placed while being processed: incoming, active, deferred and hold.
The incoming message queue holds the new mail that has been received. Each message is identified with a unique file name. Messages in the incoming queue are moved to the active queue when there is room in the active queue. 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 or moved to another queue.
Messages that cannot be delivered for some reason 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. You can change the default MTA value for
bounce_queue_lifetime from the
zmlocalconfig CLI.
Before the bounce queue lifetime sends the message back to the sender, senders can be notified that the message they sent is in the Deferred queue and has not been delivered. This is set up from the
zmlocalconfig CLI. The following attributes are configured to send a warning message to the sender:
The hold 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 hold queue.
