User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Delegates
User
HKCU\software\policies\microsoft\office\16.0\outlook\preferences
This policy setting controls whether messages sent from a shared mailbox are saved to the Sent Items folder of the shared mailbox. By default, messages sent from a shared mailbox aren't saved to the Sent Items folder of the shared mailbox. If you enable this policy setting, messages sent from a shared mailbox will be saved to the Sent Items folder of the shared mailbox. If you disable or don’t configure this policy setting, messages sent from a shared mailbox won’t be saved to the Sent Items folder of the shared mailbox.