User Configuration > Administrative Templates > Microsoft Outlook 2016 > Security > Cryptography
User
HKCU\software\policies\microsoft\office\16.0\outlook\security
This policy setting allows you to require that all e-mail messages be encrypted when sent from Outlook. If you enable this policy setting, the Encrypt button is automatically selected on all outgoing e-mail messages, meeting invitations, and other Outlook items. Users must select an appropriate certificate to encrypt the message for the intended recipient. If you disable or do not configure this policy setting, outgoing e-mail messages are not encrypted. If you disable this policy setting, users will not be able to change the configuration.