Sending email to a forwarded address gives "550 5.1.1 User doesn't exist"

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 was previously on the cPanel account but was removed improperly. The email address will not appear on the "Email Accounts" page in cPanel, but its mail directory still exists.

Workaround

1. Access the server's command line via SSH or "Terminal" in WHM.
2. Move the forwarded email account's mail directory.

   mv -v /home/$cpaneluser/mail/domain.tld/$username{,.$(date +%s)}

   Please note that "$cpaneluser" and "$username" must be replaced with the cPanel user's username and the email account's username, respectively.

Additional resources

Email Accounts