MAPI is an acronym for Messaging Application Programming Interface. It is a standardized set of C functions placed into a code library known as a Dynamic Link Library (DLL). The functions were originally designed by Microsoft, but they have received support of many third party vendors.
Having a standard library of messaging functions allows Windows application developers to take advantage of the Windows messaging subsystem, supported by default with Microsoft Mail or Microsoft Exchange. By writing to the generic MAPI interface, any Windows application can become "mail-enabled". Since MAPI standardizes the way messages are handled by mail-enabled applications, each such application does not have to include vendor-specific code for each target messaging system.
The MAPI library is also available to Visual Basic application writers through a Basic-to-C translation layer.