I am checking PidlidPrivate property in MAPI.
Based on https://learn.microsoft.com/en-us/office/client-developer/outlook/mapi/pidlidprivate-canonical-property, this property can hide the message.
So I use MFCMAPI to set the value of PidlidPrivate property to True. But that does not work. The message will always show.
I test in Outlook 2019.
No, the message will not be hidden. It is only applicable to appointments from other user's calendars - if an appointment is marked as private, Outlook won't show its details (only start/duration). All of its properties are still accessible through MAPI.
If you want a message to be invisible to an end user, create a hidden (associated) item. Outlook Object Model exposes messages like that through MAPIFolder.GetStorage. You can see these messages in OutlookSpy (I am its author) if you click IMAPIFolder button and go to the "Associated Contents" tab. It will also let you edit MAPI properties (click IMessage button, double click on a property, etc.)