User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Other
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\general
This policy setting controls whether Outlook is the default program for e-mail, contacts, and calendar services. If you enable this policy setting, the "Make Outlook the default program for E-mail, Contacts, and Calendar" check box on the General tab of the Office Center is selected and users cannot change it. If you disable this policy setting, users cannot make Outlook the default program for these services. If you do not configure this policy setting, Outlook is made the default program for e-mail, contacts, and calendar services when it is installed, although users can designate other programs as the default programs for these services.