2.2.2.18 PidTagParentKey Property
Type: PtypBinary ([MS-OXCDATA] section 2.11.1)
The PidTagParentKey property ([MS-OXPROPS] section 2.861) contains the search key that is used to correlate the original message and the reports about the original message. The server sets the property on the report message to the value of the PidTagSearchKey property ([MS-OXCPRPT] section 2.2.1.9) of the original e-mail message.