User Configuration > Administrative Templates > Microsoft Outlook 2016 > Security > Cryptography
User
HKCU\software\policies\microsoft\office\16.0\outlook\security
This policy setting controls whether Outlook checks for the user's private key when the user sends an encrypted email and the user is included as a recipient of the email. The user can be included as recipient either directly or as a member of a distribution list. If you enable this policy setting, Outlook checks for the user's private key. If you disable this policy setting, Outlook doesn't check for the user's private key. If you don't configure this policy setting, Outlook checks for the user's private key.