Windows Mail is fully compatible with Simple MAPI messaging functionality. Simple MAPI is a set of functions and related data structures that can be used by software developers to add messaging functionality to their applications.
To register Windows Mail as the default MAPI client, see Make Windows Mail the default e-mail program.
MAPI functionality is widely used by standard Windows applications, mainly for sending documents using default e-mail program, e.g. Windows Mail. See examples below: