Some client cant open Exchnge Info Store

Running Exchange 5.5 on a Windows 2000 pure Active Directory environment. Clients are Windows 2000 Pro. using Outlook 2000. System has been up and running just fine for nearly 2 years. Recently, some users log in on their workstations to access their exchange outlook email. Error message indicates that Exchange server cannot be accessed, and information store cannot be opened. Normally this would indicate a DNS resolution problem, IP problem, or exchange server service problem. DNS info is correct and has not been changed recently. The IP information is correct, workstations in question CAN ping DNS servers and the exchange server, also, can ping using the host name. So IP and name resolution is ruled out. users can still use domain and other network resources, outlook web still functions for them. please note that other users are still using outlook/exchange normally without any problems whatsoever. this problem is happening to a handful and are in the minority. There are no errors in the error logs, neither on the server, nor on the workstations. the problem virtually doesn't exist! yet these staff members can't open their mailboxes. looking on support pages has yielded no solutions but the typical problems (the things i listedabove).

absolutely out of ideas and energy. any help or point in the right direction would be most appreciated.