User Configuration > Administrative Templates > Microsoft Outlook 2016 > Security
User
HKCU\software\policies\microsoft\office\16.0\outlook\security
This policy setting allows you to specify whether replies will be automatically signed. If you enable this policy setting, the option to respond automatically to a signed message with a signed response will be overridden, and an unsigned response will be the default reply to a signed message. If you disable or do not configure this policy setting, a signed response will be the default reply to a signed message.