The mail server could not deliver mail to [email protected] The account or domain may not exist, they may be blacklisted, or missing the proper dns entries. – Knowledgebase

All of sudden emails are getting bounced with below bounce back messages on the cPanel server as below –

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

 

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