Telnet to port 25

I’m experiencing difficulty connecting to my mail server by telnet on port 25. This problem only occurs if I connect from the LAN, in which case the connection is dropped instantly. If I connect from the internet to the firewall IP, the connection is successful. Server is Exchange 5.0(SP2) on NT4(SP6a). TCP Port 25 is forwarded from firewall/router. Any ideas?