Using Classic Outlook on Windows for personal email, calendar, and contact management
Not sure why Microsoft have not supplied answer in this thread, but copied from another is answer. It works.
I used zx3junglist answer in a different way. I found a regedit entry for:
Computer\HKEY_CLASSES_ROOT\AppUserModelId\Microsoft.Office.OUTLOOK.EXE.15
But I am using Outlook 2016 (should be OUTLOOK.EXE.16, not 15)
I renamed Computer\HKEY_CLASSES_ROOT\AppUserModelId\Microsoft.Office.OUTLOOK.EXE.15
to
Computer\HKEY_CLASSES_ROOT\AppUserModelId\Microsoft.Office.OUTLOOK.EXE.16
and popups work. When I click the popup, I get the email!
I hope this helps some!