2.2.2.4.2 PidTagMessageClass Property

Type: PtypString ([MS-OXCDATA] section 2.11.1)

The PidTagMessageClass property ([MS-OXCMSG] section 2.2.1.3) specifies the type of the Message object. A Personal Distribution List object MUST have this property set to either "IPM.DistList" or a string prefixed with "IPM.DistList".