User Configuration > Administrative Templates > Microsoft Word 2016 > Word Options > Security > Trust Center > Protected View
User
HKCU\software\policies\microsoft\office\16.0\word\security\protectedview
This policy setting allows you to determine if Word files in Outlook attachments open in Protected View. If you enable this policy setting, Outlook attachments do not open in Protected View. If you disable or do not configure this policy setting, Outlook attachments open in Protected View.