[44net] SMTP port 25

Is anyone running a SMTP server on their gateway? Most ISP's block port 25 outgoing, the problem I am seeing. I see a email handshake come in and forward over the wireless LAN, But when it comes from a non 44 IP is tries to reply using my assigned ISP provided IP and obviously that doesn't work. So is there some way with IP tables or something else to set and advanced IP rule where is it's a destination on port 25 to route thru amprgw.sysnet.ucsd.edu using my 44 address?