There are a lot of emails in Home > Tools&Settings > Mail Server Settings > Mail Queue that are not being processed.
Checking of the mail queue shows the following reason why emails 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.
Sender IP address is not in the public blacklists. Can be checked here: mxtoolbox.com
Sender IP address has a bad reputation in reputation centers, for example, in talosintelligence.com by Cisco.
Server’s IP addresses are blacklisted on recipient’s side or in the reputation centers which are used by a recipient mail servers.
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
Contact a recipient with a request to remove affected IPs from recipient’s mail server blacklist.