[blacklion]

API is documentation for MAPI.DLL in this case. It is useless under non-windows system and requires this DLL, which is, effectively, Outlook backed.

Network protocol between MAPI.DLL and Exchange server newer was published (but partially revers-engineered).

And I've participated in project which uses MAPI.DLL, it was buggy as hell, and we delayed product release for whole year (!), trying to workaround all bugs. It was complete disaster and failed product too late to market.