User Configuration > Administrative Templates > Microsoft Outlook 2016 > Security
User
HKCU\software\policies\microsoft\office\16.0\outlook\security
By default, third-party ActiveX controls are not allowed to run in one-off forms in Outlook. You can change this behavior so that Safe Controls (Microsoft Forms 2.0 controls and the Outlook Recipient and Body controls) are allowed in one-off forms, or so that all ActiveX controls are allowed to run.