So, the Exchange team decided they didn't want to maintain the MAPI documentation anymore since they don't ship MAPI anymore. So the Outlook team stepped up and took over the docs. As such, you can now find them under the Outlook branch in the MSDN left-nav.
Steve Griffin has been working with the Outlook team on prettying them up and updating them for a few months, and they're now better than ever! He even got a new icon for MFCMAPI out of the deal!