On servers running cPanel, some times it is found that mail sent to valid users is bounced back by the mail server.

The bounce back messages will be similar to the following.

If the email account does indeed exist, then it is need to run the following commands to correct the issue.

Also check the /etc/localdomains file and verify that the domain name is present. Also verify that the DNS line in the
/var/cpanel/user/username contains the domain as well.