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. 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
1. Log in to Plesk control panel
2. Go to Tools & Settings > Mail Server Settings
3. Click on the Mail Queue tab.
4. Those undelivered messages will be listed in the messages in the mail queue section.
The Clear 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 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 and do suggest contacting the server-team support for assistance to check further on the reason.