On servers running cPanel, you may find that mail sent to valid users it bounced back by your mail server. The bounce back messages 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.
H=(nf-out-0910.google.com) [IP] F=<firstname.lastname@example.org> 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.
You will then also need to check your /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 as well.