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MAPI was created to provide a standardized application-level interface that allowed messaging components to communicate with widely incompatible messaging systems. Simple MAPI, or MAPI version 0, is a very limited library of only 12 functions written by Microsoft, which enabled the Microsoft Mail client to communicate with Microsoft Mail post offices. The extended MAPI, or MAPI version 1, specification was written by Microsoft in collaboration with more than 100 software vendors, and is thus an open specification that allows MAPI-compliant systems to communicate freely with other MAPI-compliant systems. It completely supersedes the older version.

The extended MAPI specification includes definitions of various external components, descriptions of what they do, and a list of the interfaces they expose. The interfaces between components are not APIs, but COM interfaces. COM components are bound together to form applications and can be extended to develop custom-application extensions. All MAPI components use or implement MAPI objects, and all MAPI objects are COM objects.

MAPI is available on all Microsoft Windows®–based desktops. The Windows Messaging Subsystem (WMS) is the MAPI standard user interface. WMS originally was referred to as the Exchange client, but was renamed to alleviate confusion. As a complicated structure, MAPI offers a number of different architectural perspectives:

* The component model consists of layers of components that call each other through the mediation of MAPI32.dll, which is loaded as part of the logon process. This intermediary layer acts as a broker between the client applications, service providers, and MAPI itself.
* The process model examines MAPI as a collection of processes on the desktop.
* The object model is a hierarchy of objects that MAPI offers to its clients from the system COM library by accessing the root Session object, thus gaining access to the rest of the object model.

Microsoft Exchange Server embraces Internet standards and is a MAPI service provider. It utilizes MAPI notification to register events and includes three types of services: message transport, message store, and address book or address list. When a new message is detected on the system, the transport provider alerts the MAPI spooler, which processes the message and copies it to the default message store. The address book freely provides client access to directory information—a list of known recipients that may be either single users or distribution lists.

When Microsoft Exchange Server sends a message, it relies on the MAPI subsystem to direct client requests to one or more transport-service providers. Microsoft Windows dynamic-link libraries (DLLs) serve as drivers for each transport provider. They also provide the interface between the MAPI spooler and the underlying messaging system or services.

Exchange Server will continue to support the widest possible array of network protocols (such as RPC, IMAP4, POP3, NNTP, HTTP, and so on), and it will continue to support MAPI as a programming interface.

http://msdn.microsoft.com/library/de..._outextend.asp

http://developer.netscape.com/docs/m...r/mapisupp.htm
http://www.wesleyan.edu/its/email/pc-eudora/apF.ctt
http://www.amazon.com/exec/obidos/tg...59800?v=glance