Brilliant! Rated 5 out of 5 stars

I don't take time to review typically, but this is a fantastic add-on, esp. for developers, researchers, and lots of folks who'd rather have full control over the layout of their HTML e-mails and don't have time to mess with 1/2-baked HTML editors and clunky toolbars. I have wondered for years now why not more e-mail clients would embrace this feature.
I would vote for adding support for **Markdown Extra** syntax, for tables, footnotes, ToC and abbreviations -- but it's a great functioning add-on as it stands.
Thank you to the author.

This review is for a previous version of the add-on (2.5.0.1-signed). 

Thanks, and Markdown Extra

Thanks a lot for the review!

Markdown Here depends on Marked.js for Markdown rendering, so support would have to be added to it for Markdown Extra. There is an issue for it (https://github.com/chjj/marked/issues/27) discussing exactly that. Fingers crossed.