Outlook can not find Excahnge server

I have MS SBS 4.5 and exchange server 5.5 with SP2. Exchnage server gets e-mail from ISP mail server. All user can send and receive e-mail except when Exchange server can not connect to ISP mail server. Some times ISP Internet connection goes downand that time outlook user can not find my exchange server eventhough they are logged in to the NT server OK. Why User can not open Outlook without any problem when Exchange server can not find ISP’s mail server? User can not send receive e-mail but at least outlook should connect to exchange server which is on LAN.