w98 dialup for exchange access w/outlook

I have a Windows 98 laptop client that previous to 5/4/02 had no problem dialing up and accessing the exchange server using outlook 98. When connecting with network card,user logs into domain and has no problem with outlook/exchange connection. Withdialup, clicks cancel for logon screen to domain. Then uses dialup through a shiva modem that gets user to network. Gets an error message that there is no domain controller to authenticate user and so some network resources won’t be available.Apparently exchange server is one of these! Can get to internet but not to exchange server. User says he changed nothing. Any ideas how to get the boss’s laptop working again? If you need more details, please ask.