Symptoms
Sending an email to a forwarded email address will result in a bounce-back message, even though the email message was forwarded to the intended destination. Error messages similar to the following can be found in exim_mainlog.
LMTP error after RCPT TO:<username@domain.tld>: 550 5.1.1 <username@domain.tld>
User doesn't exist: username@domain.tld
Description
This problem occurs when the forwarded email address existed on the cPanel account previously, but was removed improperly. The email address will not appear on the "Email Accounts" page in cPanel, but its mail directory still exists.
/home/cpaneluser/mail/domain.tld/username/
Workaround
To prevent the bounce-back message, remove or rename the forwarded email account's mail directory.
mv -v /home/cpaneluser/mail/domain.tld/username{,.bak}