Simple MAPI is a subset of 12 functions which enable developers to add basic messaging functionality. Extended MAPI allows complete control over the messaging system on the client computer, creation and management of messages, management of the client mailbox, service providers, and so forth. Simple MAPI ships with Microsoft Windows as part of Outlook Express/Windows Mail while the full Extended MAPI ships with Office Outlook and Exchange.
In addition to the Extended MAPI client interface, programming calls can be made indirectly through the Simple MAPI API client interface, through the Common Messaging Calls (CMC) API client interface, or by the object-based CDO Library interface. These three methods are easier to use and designed for less complex messaging-enabled and -aware applications. (Simple MAPI and CMC were removed from Exchange 2003.)
MAPI was originally designed by Microsoft. The company founded its MS Mail team in 1987, but it was not until it acquired Consumers Software Inc in 1991 to obtain Network Courier that it had a messaging product. Reworked, it was sold as MS PC Mail (or Microsoft Mail for PC Networking). The basic API to MS PC Mail was known as MAPI version 0 (or MAPI0). MAPI uses functions loosely based on the X.400 XAPIA standard.
The full Extended MAPI interface is required for interfacing messaging-based services to client applications such as Outlook. For example, several non-Microsoft e-mail server product vendors created "MAPI service providers" to allow their products to be accessed via Outlook. Notable examples included Zimbra, HP OpenMail, IBM Lotus Notes, Zarafa, and Bynari.
MAPI0 also had a service provider interface of sorts. Indeed, Microsoft used this to interface MS Mail to an email system based on Xenix, for internal use.
Extended MAPI is the main e-mail data access method used by Outlook, to interface to Microsoft Exchange, via MAPI service providers shipped with Outlook.
"MAPI protocol" is a colloquial name for the MAPI/RPC. At times, Microsoft has also called it "Exchange RPC" and "Outlook-Exchange Transport Protocol".