Unreachable port on WAN adapter

I have a win2k server which has two NICs, LAN and WAN. The WAN adapter has
an address in the range of the seperate hardware firewall, and the LAN
adapter has an address in the range of the workstations, which use the
server as the gateway. That's all working fine. However, where the exchange
server accepts incoming SMTP connections on the WAN address no problem,
Remote Administrator (standard port 4899) won't connect. I can connect to
Radmin using the LAN address, but not the WAN, either from within the
office network using a pc on the firewall range, or from the outside world
(port forwarding is set up on the firewall). Radmin shows as listening on
both addresses from the system tray icon, and 0.0.0.0 shows as listening on
the correct port if I do a netstat. What could be wrong here?

TIA,

Andy