Topic: redirect internet traffic based on protocol

[g00s3m4n]

hello there everyone..

i have two gateways on my network and i use an SME box as a proxy server.

if i set my server up to use gateway A as its default, then obviously all traffic will go out via that gateway, what I would like to do is route SMTP traffic through the other gateway. i know that i could do this on my cisco router, but i am looking to create a physical firewall and dont want traffic coming back in just to go  out again (if that makes any sense to you!)

any suggestions?

thanks

g