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=<emailid@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
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.