I'm not sure this is the best way to do it, but I would imagine you could do it via iptables. Tell your mail server to bind to all interfaces, and then let iptables handle the the whole interface issue. You could use iptable's nat prerouting and postrouting to accomplish the task.
Let me point out, I am not an iptables expert. I know enough to do simple firewalling, but that's about it. I do know iptables can do stuff like this... I have my server's iptables DNAT ssh requests from a ridiculously high port (port forwarded that way on our NAT gateway/router) for security purposes, to port 22.