Why are BCC recipients not migrated from older versions of Exchange?
We cannot migrate BCC recipients from older versions of Exchange because Microsoft's Exchange Web DAV API does not return this information.
Read Microsoft's article on MSDN: http://blogs.msdn.com/b/webdav_101/archive/2008/04/24/why-can-t-webdav-read-bcc.aspx
Note: Exchange Web DAV also stores the list of resources/optional attendees in the BCC field of appointments. Therefore the list of resources/ optional attendees cannot be migrated for appointments because Exchange's own API does not return this information. Refer to the following message: