Unless I’m mistaken, currently the Email only card is Wsiwyg only. Meaning you can type in text, add links, images, etc.
However it doesn’t seem possible to have an Email only card that is also html card.
So for example if I wanted to include a button at the end of an email like
<button type="button">Share this email with a friend!</button>
there doesn’t seem to be a way to do this without also putting that button into the post on the site.
I see the email call to action card feature was added, which looks great and fulfills the goal above. However, I would definitely still appreciate the original “newsletter-only HTML card” feature request so I could add tables within an email card.
Likewise, I have seen some others asking for a desktop-only card which would be really valuable as well. Sadly I am out of votes 
An alternative would be to allow emailing a post that would be hidden on the website while linking to another desktop-formatted page instead.
I’d like to see that too. Generally the function to include html in other cards. This would enable to format a proper email footer.