I have this problem and i have spent the last couple of days searching google and other sites to try and find an answer but each time i just end up getting no where or more confused than when i started.
The problem i have is we have setup a free e-mail site and the users get a free POP3/IMAP account but because of the way cPanel/WHM is setup i believe this also gives them access to the SMTP server.
Because of spammers i want to disallow access for these users but cant find anything on it. I'd either like to ban them per username or per domain name. Is there anyone here that can advise me on this?