Question: My Exchange Server 2003 (running on Windows Server 2003) fails at around 04:54 on a regular basis, though not necessarily every day.

By "fails" I mean that my colleagues try and check their emails and Outlook says "outlook is not connected to exchange." Since outlook tries to update the email every 3 minutes or so, and it records the time when the folder was last updated, it is possible to see the time of failure.

It is impossible to download emails until the server is restarted. Thereupon everything works well.

I have looked in Scheduled Tasks and can't see anything pertinent.

Does anyone have any ideas?

Answer: 
Try looking in the Event Viewer if something occurs around the time it fails each time. 
Hit Start -> Run -> type eventvwr in the Run box to open the Event Viewer, check the events under System and Application logs.
In addition, if you have a monitoring application such as HP openview or Centerity you can create a business service which will include all components that are required for the server to run, such as disk, cpu, memory monitors as well as networking, storage, application monitors, through that you will be able to identify the source for the server’s fail on that specific time when it fails.
Yeah, SP2 might do it. If you still have issues, Google for "exchange 2003 sp2 hotfix" and see if any of them are relevant to your problem. Another workaround is to set the Exchange Information Store to automatically restart after a failure in services.msc (if that's the service that's failing), but this doesn't fix the root cause of the problem.