OWA Security Information

We have NT4 network with 6 servers. Running Exch 5.5 sp3 on NT 4 box w/sp4. This box has dual nic’s (no IP forwarding). Have installed IIS 4.0 and OWA – all is working fine. Looking for recommendations/experience with keeping the system secure. Server is a Proliant 1600r P2/266 supporting about 80 users. Thanks Jeff