User Configuration > Administrative Templates > Microsoft Outlook 2016 > Miscellaneous
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\mail
This policy setting determines whether users can send mail through Simple MAPI when Outlook is not active. If you enable this policy setting, users are required to have Outlook running to send mail through Simple MAPI. This will force users to open Outlook to send mail, ensuring that Outlook Add-ins run properly before the mail is sent. If you disable this policy setting, users are allowed to send mail through Simple MAPI regardless of Outlook running. When this policy setting is not configured, it functions as if it has been disabled, allowing users to send mail through Simple MAPI without having Outlook running.