MAPI - Messaging Application Program Interface

Messaging Application Program Interface is a Microsoft proprietary protocol designed to send emails in a way IMAP does. The protocol allows 'Outlook' to fully utilize the functionality of Microsoft exchange server like syncing emails, contacts, calenders and folders.

As it is a Windows proprietary protocol, it works only with windows applications like Outlook. The distinguishing feature of MAPI is, it allows you to send an email from a windows system by 'attaching the document you are working on'.

MAPI could be understand as the 'superset' of IMAP.