nt routing

I am administering a private NT4 network.
About 1/4th of the pc on my network are able to access the pop3 server we use on a remote network. The other 3/4ths can't find the server, can't ping its ip. They can ping other machines within the lan, and they can ping the gateway address (this is a lucent pipeline 50 isdn to ethernet router). They are unable to ping outside of the lan. i have tried to discern any relevant differences between the stations that can get out and those that can't, but cannot find any. -What makes this even more perplexing is that all stations are able to access the internet via Microsoft Proxy Server.


This is becoming quite a problem, as you can imagine. Any help would be greatly appreciated.


------------------
thanks.
javier