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.