PidTagContentUnreadCount Canonical Property
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Contains the number of unread messages in a folder, as computed by the message store.
This property contains the number of messages in the folder contents table for which the MSGFLAG_READ flag is not set in the PR_MESSAGE_FLAGS (PidTagMessageFlags) property. The PR_CONTENT_COUNT (PidTagContentCount) property contains the total message count for the folder. The PR_CONTENT_COUNT and this property are read-only to clients.
Some client applications display the heading row of a category differently depending on the value of this property. For example, a client can display a category that includes unread messages in bold. This property cannot be used as a category and an attempt to do so results in the MAPI_E_INVALID_PARAMETER value being returned from the IMAPITable::SortTable method.