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MAPI is a Microsoft API that helps ensure system independence for messaging applications. It has become widely supported in the industry. MAPI provides a layer of functionality between applications and underlying messaging systems, helping developers create products that are compatible with a wide range of systems and platforms. MAPI is often called "messaging middleware," and, like other middleware products, it helps to promote the creation of products and speed their release to market.

The MAPI architecture has three layers. At the top are MAPI-compliant applications. In the middle is the MAPI subsystem, which has programming interfaces and a service provider interface. The MAPI service providers are at the bottom. They include store providers, message transfer agents, and address book providers. MAPI clients communicate through the MAPI subsystem to MAPI service providers. The MAPI service provider performs the requested action for the client and passes the action back through the MAPI subsystem to the MAPI client. It is only necessary to make a client application MAPI compliant in order for the application to access the service provider functionality. A specific interface is not required for each provider. This is similar to the way in which applications that use the Microsoft Windows printing subsystem do not require drivers for every available printer.

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