2.2.1.86 PidTagOriginalMessageClass Property
Type: PtypString ([MS-OXCDATA] section 2.11.1)
The PidTagOriginalMessageClass property ([MS-OXPROPS] section 2.831) is included in reply or resend messages to designate the value of the PidTagMessageClass property ([MS-OXCMSG] section 2.2.1.3) from the original message.