FAQ: Indexing Uncached Exchange mailboxes
WDS 3.0 and Windows Vista can index uncached Exchange mailboxes, but this functionality is disabled by default. It needs to be enabled via group policy.
If you’re not in a group-policy managed environment (or your administrator does not set this setting), you can enable it by creating a DWORD registry key in:
called “PreventIndexingUncachedExchangeFolders” and setting it to a value of 0.