3.1.4.2 Opening a Document Object

When a user opens a message, the client opens the Message object as specified in [MS-OXCMSG] section 3.1.4.1. The client determines the message type by examining the PidTagMessageClass property ([MS-OXCMSG] section 2.2.1.3), as specified in section 2.2.2.1.

If the value of PidTagMessageClass does not begin with "IPM.document.", the message is not a Document object, and the client handles the message in a way that is appropriate for that particular type of Message object. If the value of the PidTagMessageClass property does begin with "IPM.document.", the message is a Document object, and the client retrieves the attachment as specified in [MS-OXCMSG] section 3.1.4.11. If there are zero attachments, the client displays an error. If there is more than one attachment, the client can either display an error or pick one of the attachments. For details about attachments to a Document object, see section 2.2.2.3. When a Document object is opened, the client can open the message's underlying attachment directly, thereby behaving in the most optimal fashion from a user's perspective.