2.2.7.1 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 begin with "IPM.Schedule.Meeting.Resp" and MUST be appended with either ".Pos", ".Tent", or ".Neg", indicating accept, tentatively accept, or decline, respectively.