Policy
Set default search scope
This policy allows you to specify the default search scope to be used. Users will not be able to change the default once this policy is set, but they can change the scope while running a search. If you enable this policy, you can specify the folders that Outlook searches by default when the user begins a new search. If you disable or do not configure this policy, Outlook searches all folders in the current mailbox when the search is initiated from the Inbox. If the search is initiated from another folder, the search only includes items from that folder. Users can still change the scope when searching.
49f035c8beff L_SetDefaultSearchScope Registry
Copy registry mappings
HKCU\software\policies\microsoft\office\16.0\outlook\search\defaultsearchscope Policy notes
This policy allows you to specify the default search scope to be used. Users will not be able to change the default once this policy is set, but they can change the scope while running a search. If you enable this policy, you can specify the folders that Outlook searches by default when the user begins a new search. If you disable or do not configure this policy, Outlook searches all folders in the current mailbox when the search is initiated from the Inbox. If the search is initiated from another folder, the search only includes items from that folder. Users can still change the scope when searching.