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 requires digital signatures on all outgoing e-mail messages. If you enable this policy setting, Outlook requires all outgoing messages to be digitally signed before being sent. If you disable or do not configure this policy setting, Outlook does not require outgoing messages to have digital signatures.