MSG files can contain other types of items than just emails. Some of the other types, and their message classes are:
Meeting requests (IPM.Schedule.Meeting.Request)
At present the Viewer does not display these items like they are shown in Outlook, and vital information is missing. For example,
Appointments do not show Start time, End time, and Location
Contacts do not show the person's contact numbers, addresses, etc.
BTW https://ideas.accusoft.com/ideas/PDV-I-627 is similar to this but only mentions meeting requests.