User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Mail > Compose Messages
User
HKCU\software\policies\microsoft\office\16.0\outlook\options
This policy setting enables you to force users to select an e-mail account from which to send outgoing e-mail. If you enable this policy setting, users must choose an e-mail account before they can send an e-mail. If you disable or do not configure this policy setting, e-mail is sent from the default e-mail account if users do not select a specific e-mail account.