Hi Winan,
Yes I've seen your message but wasn't quit sure what you were doing. When you set up an email user I would use linuxconf. Make sure you assign all email user to the mail group. This allows them to use the mail service to send/receive mail, but they will not be able to log into the server locally or otherwise. You could then always check all user assigned to the mail group in linux configure, and like Bob Lee said thier email address would be the username@mailserver.com.
It time to break for lunch. I'll check back later.