Many of our customers are still using MAPI for programmatic access to Exchange mailboxes. The documentation for MAPI was recently relocated on MSDN, so we’re trying to get the word out about where to start.
MAPI Documentation on MSDN (http://msdn.microsoft.com/en-us/library/cc765775.aspx)
For more details on the change in content and organization, head over to the Office Client Developer Content Blog.