SMTP Gateway

I have an Exchange server and I would like to protect it by adding an SMTP/POP3 gateway on the DMZ of the firewall. Any recomendations, I am looking at a Barracuda, but I am thinking this is overkill for what I want to do. I know Microsoft has a solutions for this using a Windows Server, but I would like the gateway to be on a different platform either Linux or BSD.

Any Sugestions?