A Note from the Developer on Permissions
In order to get the transport message headers I have to use the EWS makeEwsRequestAsync method, which requires the ReadWriteMailbox permission level. See the article Understanding Outlook add-in permissions for more on this. If I could request fewer permissions I would, since I only ever read the one property, but I have no choice in the matter.
When REST is more widely available, and a few REST specific bugs are fixed, I'll be able to switch to REST and request a lower permission level.
Steve