Firewall Forwarding Fun (not!)

Richard Harmon writes in with this bit of networking trouble: "I'm trying to set up a linux firewall at my office, but I can't get the forwarding to work. I've tried ipportfw, but it acts like the packets don't match the masq' rules or something. I can't figure out a way to have the smtp, pop3, imap, x.400 and so on forwarded /through/ the firewall with NAT (network address translation).. I basically want my firewall to sit on the mail sever's old (outside) IP, and then when it recieves network traffic on that IP to forward it to our internal network (192.x.x.x). HELP!!!