Linux smtp forwarder DMZ to Exchange 2k

I’m in the process of bringing our internet email in-house. We are running Exchange for enterprise mail right now and I intend to host the internet mail there as well. I plan on putting a cheap linux box in the DMZ to forward smtp traffic in and out of the domain. I’m asking for recomendations for the mail server to use(sendmail, postfix, other). The Exchange box is already protected by McAfee ePolicy and Groupshield but I would also like to run some spam filtering and anti-virus on the linux box.

Any suggestions, pointers, and/or tips will be aprreciated.