On servers running cPanel, you may find that mail sent to valid user is bounced back will an error message which will be similar to the following:

PERM_FAILURE: SMTP Error (state 9): 550-”The recipient cannot be verified.
Please check all recipients of this550 message to verify they are valid.

If this is occurring, you will need to check your exim_mainlog file for entries
similar to the following.

<emailaddress@gmail.com> rejected RCPT
<emailAccount@domainName>: No Such User Here

If you are sure that email account does indeed exist, you will need to run the
following commands to correct the issue.

/scripts/updateuserdomains
/scripts/mailperm

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