2.2.1.60 PidTagSubjectPrefix Property
Type: PtypString ([MS-OXCDATA] section 2.11.1)
On an E-mail object, the PidTagSubjectPrefix property ([MS-OXCMSG] section 2.2.1.9) represents an action on the e-mail message, such as "RE: " for replying and "FW: " for forwarding. If this property is absent, there is no subject prefix for the e-mail message.
On report messages, the value of the PidTagSubjectPrefix property is set as follows for the specified types of reports and responses:
Delivery receipts: "Delivered: "
Read receipts: "Read: "
Sender response on read receipt requests: "Approved: "
Non-delivery reports: "Undeliverable: "
Non-read receipts: "Not Read: " or "read: "