Regarding External Proxy

Hi,

I have windows 2000 advanced server with Active Directory installed and 50 windows 2000 professional clients.I want to block email facility.I want no users can access email but they can open any other web site.
I heard that through the external proxy we could do this thing.When they put some IP address then they could open the mail web site and some other web site but not before that.
I want to know how to configure external proxy.
I would be highly thankful if someone provides me assistance and solution of my question.

With regards.

Anurag.