If it is behind a firewall... then you have to:
a.) bind the server to internal I/P address.
b.) make sure that the appropriate ports are open on the router.
c.) If you are sending emails out... AND the server is supposed to be the sending MX server,
then you need to make sure that you are not going to pick up the router's address but the actual external I/P address for the server. Not sure how you do this in CISCO, but in our routers I have to tell it to "preserve" outbound email address.
Finally, although your server doesn't "itself" use the external I/P address, anything inside the firewall.. should still be able to resolve the external address.