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.