Policy
Configure Simple MAPI sending prompt
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.
04f718f64e2f L_SimpleMapiSend Registry
Copy registry mappings
HKCU\software\policies\microsoft\office\16.0\outlook\security\promptsimplemapisend Policy notes
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.