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.
----- The following addresses had permanent fatal errors ----- (reason: 550 ... User unknown)...
Go to cPanel - "Mail" - "Email accounts" Find the mailbox you need and click "Password"....
You can do that at cPanel - "Email" - "Email Authentication".
Go to cPanel - "Mail" - "Email accounts". Complete all of the fields and click “create account”...
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