User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Preferences > Search Options
User
HKCU\software\policies\microsoft\office\16.0\outlook\search
This policy sets the default action in All Mail Item search to not include search results from the Online Archive. If you enable this policy setting, search results from the Online Archive will not be included in an All Mail Item search in Outlook. If you disable or do not configure this policy setting, search results from the Online Archive will be included in an All Mail Item search in Outlook.