2.2.6.1 PidTagMessageClass Property
Type: PtypString ([MS-OXCDATA] section 2.11.1.2)
The value of the PidTagMessageClass property ([MS-OXCMSG] section 2.2.1.3) MUST be "IPM.Schedule.Meeting.Request" or MUST be prefixed with "IPM.Schedule.Meeting.Request.".