I am setting up a Zimbra 5.0 server for my campus. I want all users to use the web interface to access the mail server. Can I configure the server so that it does not allow anyone to use any mail clients like Outlook, Thunderbird, Mac, Entourage, etc ?

If so, is it possible to selectively allow relay for some users and deny relay for all others ?

QMail has a setting wherein, we can deny relay for users by checking a check box in the setup.