This is probably due to your quota being full. You will need to delete some of your mail to make room for the new ones.
If this is not the problem check your mail filters and forwarders. If you still have a problem please contact our support team.
Go to cPanel - "Mail" - "Email accounts". Complete all of the fields and click “create account”...
----- The following addresses had permanent fatal errors ----- (reason: 550 ... User unknown)...
Go to cPanel: click "Mail" - "MX entry", choose "Remote Mail Exchanger". Click "Add New Record"...
You can do that at cPanel - "Email" - "Email Authentication".
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