- Plesk for Linux
A lot of mails in Home > Tools&Settings > Mail Server Settings > Mail Queue that are not being processed.
Checking of mail queue using mailq command shows the following reason why mail cannot be delivered:
554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means.
Server's IP addresses are blacklisted on recipient's side.
Make sure that the server is not a source of SPAM. Check Plesk documentation to find what Plesk features can be used to protect the server from outbound spam - Protection from Outbound Spam
After that contact a recipient with a request to remove affected IPs from recipient's mail server blacklist.