Sorry for digging out this old thread, but if i remember right (haven't done multiplayer for a long time), opening the -port is not enough. Check out this side on direct play:
http://support.microsoft.com/?scid=kb%3Ben-us%3B240429&x=13&y=12

Try to also open port 6073 on the server. I will do tests on this very soon, because I'm working on a multiplayer project again.

Hope this helps some folks searching for the right router configuration.