User Configuration > Administrative Templates > Microsoft Outlook 2016 > Account Settings > E-mail
User
HKCU\software\policies\microsoft\office\16.0\outlook\rpc
This policy setting allows you to prevent Outlook from saving user credentials using Basic Authentication. If you enable this policy setting, Outlook will not save user credentials using Basic Authentication. If you disable or do not configure this policy setting, Outlook will allow the user to save credentials when using Basic Authentication against a server. These credentials are stored as generic and retrievable by any process running with that user's rights on the machine.