User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Mail Format > Internet Formatting > Message Format
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\mail
This policy setting controls the default message format in Outlook. If you do not configure this policy setting, new e-mail messages in Outlook are formatted as HTML. If you enable this policy setting, you can set the default e-mail format in Outlook to HTML, Rich Text, or plain text. Users can choose a format other than the default when composing messages. If you disable this policy setting, Outlook uses HTML as the default e-mail format and users will not be able to change it. If you do not configure this policy setting, Outlook uses HTML as the default e-mail format, but users can choose a format other than the default when composing messages.