Using Classic Outlook on Windows for personal email, calendar, and contact management
After messing around with every solution in this page, I finally got mine to act correctly.
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!