In this case you must to do a bit change in your master.cf file:
<incoming ip>:smtp inet n - y - - smtpd
<outgoing ip>:smtp inet n - y - - smtpd
-o content_filter=dfilt:
Replacing <incoming ip> and <outgoing ip> with the IP addresses of the different network interfaces. In practice, these IP addresses could be bound to the same network controller.
You can read this reference:
http://www.paw.za.org/docs/howtos/po...o-2.html#ss2.5
Regards