If you are having problems with mail delivery, you can view the mail queues to see if you can fix the mail delivery problem. When you open the Mail Queues view, you see a snapshot of the Deferred, Incoming, Active, Hold, and Corrupt queues content at that point in time. The number of messages in the queue are displayed on the tab and you can where messages are coming from and going to.
Note: When you open Mail Queues, the information that is downloaded is cached. Messages continue to be delivered. When you refresh the queue, you may see different information. If your queues are large, we recommend that you do not refresh too often while troubleshooting the queues.
The Mail Queue panes are divided into two sections:
The Summary/Filter pane shows a summary of messages by receiver domain, origin IP, sender domain, receiver address, sender address, and, for the Deferred queue, by error type. Click on any of the summaries in the list to see detailed information by messages. You can select more than one summary at a time. These messages are displayed in the Messages pane.
The Messages pane shows individual message envelope information for the selected summaries.
At any time, you can click Refresh to update the view.
You can manage the contents of the queues, either for all messages in the summary or for a single message as follows:
Requeue messages in the queues. Message routing is re-evaluated and earlier penalties are forgotten.
Hold moves messages to the Hold queue. Messages stay in this queue until the administrator moves them.
Release moves messages from the Hold queue. Messages are moved to the Deferred queue.
Delete messages deletes the message from the server.
Postfix queue file IDs are reused. If you requeue or delete a message, note the message envelope information, not the queue ID. It is possible that when you refresh the mail queues, the queue ID could be used on a different message.
In addition to managing individual messages in a specific queue, you can flush all queued mail on a server. When you click the Flush button, delivery is attempted for all messages in the Deferred, Incoming, and Active queues.
See About MTA Queues