MAPI 
Messaging Application Programming Interface 
A Microsoft messaging architecture and client interface component built into the Windows operating system. As a messaging architecture, MAPI makes it possible for applications such as e-mail clients, scheduling and calendaring programs, and word processors to interact with messaging systems across a variety of hardware platforms. For example, using MAPI, a word processor can send documents via e-mail. As a client interface component, MAPI is the set of functions and object-oriented interfaces that makes this functionality possible.