When the Zimbra MTA receives mail, it routes the mail through a series of queues to manage delivery; incoming, active, deferred, held, and corrupt.
The incoming queue holds a new mail message that has been received. Each message is identified with a unique file name. Messages 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 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.
Message that cannot be delivered for some reason are placed in the deferred queue until they are either delivered successfully or expire and are bounced back to the sender. The reasons for the delivery failures is documented in a file in the defer queue. This queue is scanned frequently to resend the message. The default for bounce queue lifetime is five days. This can be changed from the CLI, zmlocalconfig. See the Administration Guide, CLI appendix.
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 hold queue.
The Corrupt queue holds damaged unreadable messages.
See Monitoring Zimbra MTA Mail Queues
-------------------------------------------------------------------------------------------------------
Copyright @ 2005-2017 Synacor, Inc. All rights reserved. "Zimbra" is a registered trademark of Synacor, Inc.