On some services running WHM/Cpanel you may see mail that is sent to your valid users being bounced by the mail sever on WHM.
The message returned will be similar to this:
PERM_FAILURE: SMTP Error (state 9): 550-"The recipient cannot be verified. Please check all recipients of this 550 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. H=(nf-out-0910.google.com) IP F= rejected RCPT : No Such User Here @domainname>@gmail.com>
First, verify that the email account exists in the WHM system. Once verified, run these commands:
Once this is done, check /etc/localdomains and verify that the affected domain name is present in the file.
Finally, check /var/cpanel/user/username to make sure that the DNS entry contains the same domain.