Retrieve the properties and relationships of a message folder object.The following list shows the two existing scenarios where an app can get another user’s mail folder:
If the app has application permissions.
If the app has the appropriate delegated permissions from one user, and another user has shared a mail folder with that user, or, has given delegated access to that user. For more details, see Get Outlook messages in a shared or delegated folder.
Least privileged Microsoft Graph permission to access the action via application: Mail.ReadBasic.All.
Higher Microsoft Graph permission to access the action via application: Mail.ReadWrite, Mail.Read.