A message class is stored as a string. The encoded string usually holds the MAPI-specified name of the message class. The exception is that to provide compatibility with Microsoft Mail for Windows for Workgroups 3.1, the following MAPI message classes are mapped to the following Microsoft Mail for Windows for Workgroups 3.1 message classes:
| MAPI message class | Windows for Workgroups Mail 3.x |
|---|---|
| IPM | IPM.Microsoft Mail.Note |
| IPM.Note | IPM.Microsoft Mail.Note |
| IPM.Schedule.Meeting.Canceled | IPM.Microsoft Schedule.MtgCncl |
| IPM.Schedule.Meeting.Request | IPM.Microsoft Schedule.MtgReq |
| IPM.Schedule.Meeting.Resp.Neg | IPM.Microsoft Schedule.MtgRespN |
| IPM.Schedule.Meeting.Resp.Pos | IPM.Microsoft Schedule.MtgRespP |
| IPM.Schedule.Meeting.Resp.Tent | IPM.Microsoft Schedule.MtgRespA |
| Report.IPM.Note.NDR | IPM.Microsoft Mail.Non-Delivery |
| Report.IPM.Note.RN | IPM.Microsoft Mail.Read Receipt |