User Configuration > Administrative Templates > Microsoft Word 2016 > Word Options > Advanced > E-mail Options
User
HKCU\software\policies\microsoft\office\16.0\common\mailsettings
This policy setting allows you to configure the "Use Cascading Style Sheets (CSS) for appearance of messages" option found under Microsoft Outlook's File tab | Outlook Options | Mail | Message format. If you enable this policy setting, the option is checked. If you disable or do not configure this policy setting, the option is not checked.