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If your Exchange Server 2003 Outlook Web Access (OWA)
clients receive a “503 Service Unavailable” error message, this
indicates that the Davex.dll file can’t communicate with the requested mailbox
or public folder store. There are a several possible causes for the 503 error
in OWA, for example:

  • 503
    errors occur when the System Attendant or Information Store services
    aren’t running–or they aren’t running under the Local System account.
  • 503
    errors occur when the mailbox or public stores aren’t mounted.

If you can’t start the services or mount the stores, then you’ll
need to troubleshoot these problems. However, once these issues are eliminated
as a cause, you’ll have some real digging to do. Here are some more 503 error
causes to examine:

  • 503 errors occur when the HKEY_CLASSES_ROOT
    registry hive has a key that exceeds 259 characters.
  • 503 errors occur when there is an invalid Access
    Control List on one of the subkeys. This problem will normally show itself by
    logging events 9014, 9095, 9096, 101, 9542, 8206, and / or 10002 when the
    Exchange services are restarted.

If you suspect that this last item
is the culprit, Microsoft Product Support Services (PSS) has a tool called
HKCRScan.exe that’s developed specifically to find these kinds of registry
problems. For additional information about how to troubleshoot the 503 error in
OWA, check out Microsoft’s
Knowledge Base 823159