Is BPS installed on its own server or on exchange?
What version exchange?
What version CDO.DLL is installed on BES and exchange?
KB04912 - How to check if and where the CDO.dll file is registered on the BlackBerry Enterprise Server
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Use search on Exchange to find it and dont forget to look at hidden files during your search.
KB05270 - How to troubleshoot one-way wireless calendar synchronization issues
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