MAPI Properties

Description: Contains a number that uniquely identifies the attachment within its parent message.
Property ID: $0E21
Property type: PT_LONG
Area: Message attachment

Message stores generate and maintain this property. The attachment number is the secondary sort key, after the rendering position, in the attachment table.
PR_ATTACH_NUM is used to open the attachment with the IMessage::OpenAttach method. Within a client application's session, the PR_ATTACH_NUM property of a message attachment remains constant as long as the attachment table is open.
The message store propagates changes to the table using the Imessage::CreateAttach and Imessage::DeleteAttach methods. At its option the message store can generate table notifications on open attachment tables so that clients can resynchronize to those changes.