outlook from windows 98 laptop dialup

I have a Windows 98 laptop that connects to the network both through a network card and a dialup modem.The user can get to outlook as expected when connected via the network card. When he uses dialup he gets to the network. When he tries to open Outlook he gets the message: “Your Microsoft Exchange Server is unavailable”. I know for a fact it is available. This functioned properly prior to April 24, 2002 for the past 3 years.He says he didn’t change anything, it just stopped working. I’m inclined to believe him because he is quite p.c. literate. I tried various fixes.Obviously I’m overlooking something.
Please help asap as this is the boss’s laptop!