Debian 2.2 IPChains

I am looking to get rid of our aging NT Server. With all the viruses out there recently I am looking into a linux solution. Is there a program out there that mimics Exchange Server for linux? I currently have exim installed on the linux box for an email MTA.

Also I want to set this mail server up to also be our gateway for our users to the internet. Where can I find good instructions to setting up a proxy/firewall using linux?