If you view the IIS logs for the Web site, you see that the 404 error is returned after the HTTP GET request is redirected to the Exchange/USA/Username folder. The USA folder is only used in Exchange Server 5.5, and it does not exist in Exchange 2000.
This behavior can occur if Exchange 2000 Setup did not remove the existing Exchange Server 5.5 version of the Exchfilt.dll file from the list of ISAPI filters that are loaded by the default Web site. The old Exchange Server 5.5 Exchfilt.dll file automatically tries to redirect the user to the USA folder.
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