Require SMTP AuthenticationFor WHM version 11.30
By default, WHM uses POP before SMTP authentication. This means that when a user on a POP mail server authenticates a session, he will be allowed to send messages for 30 minutes without re-authenticating through SMTP.
While this method is secure, some administrators prefer to use SMTP authentication at all times. To require SMTP authentication, issue the following command as root in a terminal session:
Note: The same applies for IMAP servers. If you're running an IMAP mail server and you wish to require SMTP authentication when messages are sent, you will need to run the command listed above.