Provide Add-Ins web hook for User login and logout/disconnect
An optional URL could be placed in an Add-In's manifest file that would be called when the user of the Add-In logs in and logs out or becomes disconnected. This would occur automatically without the user having to invoke the Add-In. In the case of the Outlook desktop app, it would be when the user starts and quits the program.
In my situation, I need to be notified when an email has been moved to a different folder so I can determine its new message ID, even when they haven't clicked on my Add-In's UI. My web hook could be used to subscribe to an Outlook Notification.