How do I setup an email proxy on a Pix firewall?

Ladies and Gents,
I bought this anti spam proxy thingy that is supposed to filter all my incoming emails and then forward them to my exchange server. I can port-forward my incoming smtp traffic to the proxy without any problem. How do I use the same ip address for the inbound mail proxy and for my exchange server's outgoing traffic? I also have another IP address that is doing port-translation for the rest of my network.