A MAPI form is a viewer for a message. Every message has a message class that dictates the particular form that is used as its viewer. MAPI defines a few message classes and has implemented the forms for viewing messages of these classes. Client application developers can create new message classes and custom forms for viewing messages created with the new classes.
Every custom form implements a set of standard menu commands, such as Open, Create, Delete, and Reply and a set of commands that are specific to the particular form. Some of the form commands are integrated with the user interface of the client application when the form is active; other form commands totally replace the client commands.
The relationship between the MAPI components involved in using forms is shown in this diagram. Following is a brief description of the components involved; more detailed descriptions are provided in Forms.
In the diagram, notice that the form manager plays a role that is similar to other MAPI service providers, although it is not a service provider itself. That is, the form manager is a replaceable DLL that implements a portion of the MAPI interfaces. Although developers can implement their own form manager, most environments will use the form manager provided by Microsoft due to the form manager's complexity.
The components shown in the diagram and their relationship to other components are: