I amended a little rewrite. Someone else needs to review this again.

Sweet! Really like the concept.
It's already a win to have a list of registered emails.
The rendered email itself does currently only provide a small improvement from just looking at the code. Especially given the following use cases:

1. As a dev, I want a preview of email changes without the need to send one to myself
2. As an email marketing person, I want to see the rendered emails as if they were sent to me directly.

IMHO This feature would be more useful if it rendered out the html. At least the include statements should be resolved.
Also we allow to mix html and markdown formatting which leads to broken looking email previews since only the markdown is rendered.
I'm ok with treating this as a preview feature. Rendered HTML would be actually useful though.

I'd still find this more useful if somehow the html was rendered. A very good start though.