Every GUI desktop client I'm aware of embeds a real rendering engine. The difficult one is Outlook, because Microsoft made a conscious decision that compatibility between Outlook and Word was more important than between Outlook and web browsers. I believe the Word/Outlook HTML engine was based on IE 5.5.
The MSO renderer is a buggy and incomplete implementation of HTML 3.2, and I think high-DPI support is the only change of any substance at all in the last twenty years.
Apple's mail clients use Webkit under it all, basically Safari, that's why the scores are so high. It's actually an interesting story, In the book `Creative Selection`, Webkit and Apple Mail were really Chicken and Egg-like, with the two being developed for each other with the HTML Editing logic in Mail being the basis of all text fields in iPhone.
Thunderbird had the best rendering engine of them all last time I checked with the microsoft ones being worse. Even if they did embed a real rendering engine you still have to deal with all those people using office 2010