CDO & MAPI Programming with Visual Basic:

Advanced Features

The MAPI controls do offer some features beyond simply sending and receiving messages. These features are the message class, requesting a receipt, tracking a conversation thread, and displaying a recipient’s properties.

The Message Class

All MAPI messages have a message class, which is a string that broadly differentiates types of messages. The MsgType property of the MAPIMessages control gives you access to the message class. The most common message class is IPM.Note, which signifies a generic email message. If you leave the MsgType property blank when composing a message, MAPI automatically gives the message a class of IPM.Note.

A common use of message classes is to tell the receiving mail client that a custom form should be used when displaying the message. For example, if you’ve created a custom form in Outlook, Outlook uses that form when displaying any message with a message class of IPM.Note. FormName (where FormName is the name you gave to the form). Be aware that if the receiving mail client doesn’t recognize the message class, it will likely complain to the user, who may not know what the problem is.

To send a message using a custom message class, you must be sending to a recipient that is capable of receiving messages in Microsoft Exchange RTF, and the recipient’s address book entry must indicate this fact, as shown in Figure 4-7.

If you are writing an application that reads and responds to messages only of a certain class, you may want to set the ...