Is the EventLog service running correctly?
Have you manually cleared the logs? (right-click the app log -> properties -> Clear)
Have you increased the log file size? (Right-click the app log -> properties -> size)
Have you deleted the app log file? (set EventLog service to disabled, reboot, search/delete the *.evt files on the server, re-enable the service, reboot, and new files will be generated)
Have you tried looking at the event logs from a remote computer? (In Event Viewer, right click 'Event Viewer (local)' and choose 'Connect to another computer...')
Lastly, I don't know if anything in 2000 SP4 fixes any of these issues, but it might be something to try 'if all else fails'.
As for the users not able to access their mail... you could turn on diagnostics logging (on the fly) to see if that gives you more information.
Here's some info:
Enabling Diagnostics Logging
Just note, you'll be filling up the event logs with information (we're talkin thousands of events an hour if you would enable ALL diagnostics at Maximum logging.)
- rp