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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.