Exchange/Outlook network problem

NT Server 4.0 sp6a, Exchange 5.5, Windows 95/98 clients, Outlook 97, Ethernet-TCP/IP LAN connected to a WAN. Intermittently, when clients are opening Outlook they are blue screening with the message
“Network write error. You have no space available on the server.”
The system then locks. You cannot Ctrl+Alt+Del to End Task, but must do a hard shutdown.
There is plenty of disk space, and we do not have quotas. Besides, this is a blue screen and not a windows popup. Nothing is showing in Event Viewer.
I thought it might be a corrupted TCP/IP stack, but the server itself seems to have no visible network connectivity problems.
I can find nothing in the MSKB. Any ideas or KB articles would be appreciated.