NAT and Domino R5

We have two locations running Domino 5.0 servers connected together via the
internet. When we turned on NAT in a new router we were setting up, people in the other location were no longer able to send us email or open databases on our server. Replication only worked if it was initiated on our end.

We ended up reprogramming the router to not use NAT on specific IP addresses (namely the ones used by all of our servers). From a security standpoint, this unfortunately leaves the servers open toanyone on the
internet!

I am still searching for a better solution. I believe that something in the configuration of the Domino server, perhaps the TCP/IP port that it uses to talk to the other server, is the answer?