Exchange 5.5 problem

I have a client who needs to use his PC at work on some weekends but he said his inbox would not shown up when he login on a weekend. I have checked his login restriction and he is on 24/7. I check his Outlook and Exchange settings but can’t find anything related to time restriction. He is using win98 with Outlook 98. The server is NT4 with Exchange 5.5. Glad if anyone can help.