User Configuration > Administrative Templates > Microsoft Outlook 2016 > Security > Security Form Settings > Programmatic Security
User
HKCU\software\policies\microsoft\office\16.0\outlook\security
This policy setting allows you to specify what occurs when a program attempts to send mail programmatically, using Simple MAPI. If you enable this policy setting, you can choose whether Outlook always allows sending mail, always disables sending mail, or prompts users to specify whether to allow or disallow sending mail. If you disable or do not configure this policy setting, Outlook prompts users to specify whether to allow or disallow sending the mail.