A form is a custom e-mail message that, because of its distinct appearance and functionality, is rendered differently than a standard e-mail message. This difference reflects the addition of nonstandard fields used to provide the added functionality.
The way a message is rendered is controlled by its message class. In Microsoft® Exchange, standard e-mail messages have the class IPM.Note, and every new message is assigned this class by default. This means that when a person uses a Web browser to create a message, Collaboration Data Objects (CDO) renders a new message of the class IPM.Note. As with non-Web e-mail clients, the sender can then fill in the message's fields and mail it. It is sent as an IPM.Note and displayed as such to the recipient.
A new message class is typically distinguished by the presence of a different set of message fields, which provide access to MAPI properties. The new class usually contains all the fields of the standard message plus others that make it useful for its specific purpose. Regardless of a form's appearance, function, or message class, it remains a message, and Microsoft Exchange stores and transfers it like any other message.