How are you handling FOUO?

I know the obvious answer is to treat it like a type of CUI. My main question is about what kind of specific guidance I should provide to employees handling emails or documents to and from an agency that is still solely on the legacy FOUO system. Should they just follow the lead of that agency or should they remark things as CUI? Or do a blend of CUI/FOUO? There are going to be employees who ask these kinds of questions because that want to follow the rules. I'm not sure what to tell them. The guidance from this agency is nonexistent.