The "503 Service Unavailable" error is nominally a generic error message delivered by a web server when something "behind the scenes" has failed. Because the error is so nonspecific, it can be annoyingly difficult to troubleshoot, especially when the web service in question is something like Outlook Web Access (or OWA for short).

There are several basic reasons and a few more complex ones why this error may be produced with Outlook Web Access:

  • Exchange isn't running.
  • Exchange is running, but has been started under a different user account other than the local system account (making it impossible to work correctly due to permissions restrictions).
  • The mailbox OWA is trying to access hasn't been mounted or has been deleted.

One of the more complex reasons for OWA failing like this is the presence of a registry key in the HKEY_CLASSES_ROOT hive that is more than 259 characters in length. When OWA starts, it contacts the Exchange OLE DB Provider, which in turn scans through HKEY_CLASSES_ROOT for registered file types. If any of these keys are too long, the Exchange OLE DB Provider may abort. You may need to police this section of the Registry to make sure none of the key values are too long.

Another more common reason for this failure is an overly restrictive access control list on one of the subkeys in this branch of the Registry, which can also cause the Exchange OLE DD Provider to fail. Use REGEDT32 (not REGEDIT) to change ACLs

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in the Registry. (On a side note, mismatched ACLs on Registry entries are a very common cause of inexplicable or untraceable service failures and should be considered more often as a possible reason for service-related problems that don't seem to have a cause.)

Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.

This was first published in March 2003

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