exchange 5.5

-i have a w2k server and exchange server on it , and i installed a D-LINK hardware internet server , so my problem is that when any user open the ms outlook on his station the internet server opens the connection automaticaly , is there a way to keep the exchange off the internet connection and use it internaly only
-the exchange server is at the same time domain controler .
-clients are using outlook express for internal e-mail and ms outlook for internal e-mail

thanks in advance