When the Zimbra MTA receives mail it routes it through a series of queues to manage delivery. The MTA maintains queues where mail is temporarily placed while being processed: incoming, active, deferred, and hold.
The incoming queue holds a new mail message 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.
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 hold 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 can be viewed. This queue holds unreadable messages.