People are starting to find that their Outlook has suddenly changed itself to the “new Outlook” which won’t work in many environments. To force it back to normal:

HKCU\Software\Microsoft\Office\16.0\Outlook\Options\General
Add new or modify DWORD HideNewOutlookToggle = 1

HKCU\SOFTWARE\Microsoft\Office\16.0\Outlook\Preferences
UseNewOutlook = 0