Mail sent to and from your server passes through the mail queue.
Sometimes messages can get stuck in the queue, for a number of reasons.
If enough messages get stuck in the queue, it can cause your mail server
to function incorrectly. Plesk for Linux gives you the ability to
monitor the mail queue and remove messages from the queue either
individually or in bulk.
To see all the messages currently in the queue, go to Tools &
Settings > Mail Server Settings (in the Mail group) and open
the Mail Queue tab. If you need to look for a specific message,
click the 
icon to open the search toolbar. Once you
have located the messages you want to remove from the queue, select the
corresponding checkboxes and click Remove. Alternatively, clicking
Clear will remove all messages currently in the queue. The latter
option should be used with caution, as it will erase all messages
currently in the queue, both harmful and legitimate.
One scenario where this functionality comes in handy is when your
customers complain that mail they send fails to arrive to the
destination, or arrives with a significant delay. It may indicate that
spam is being sent from your server, and it is clogging the queue. Check
the Deferred queue for suspicious messages. If you find spam
messages in the deferred queue, we recommend that you set up the
Outbound Spam filter. If no spam is found in the Deferred queue, but
it still contains a lot of unsent messages, it may indicate that
something is wrong with the mail server. We recommend using the Plesk
repair tool to automatically resolve issues with the mail server.