User Configuration > Administrative Templates > Microsoft Outlook 2016 > Miscellaneous
User
HKCU\software\policies\microsoft\office\16.0\outlook\preferences
By default, categories on incoming e-mail are removed, and categories are removed when replying to or forwarding an e-mail. This setting allows you to control how categories are shared as users exchange e-mail messages. You can specify that categories are not removed for users' incoming e-mail. You can also specify that e-mail messages that users reply to or forward retain the categories on the original message.