This is a very interesting problem.
At the moment we simply ship the raw markdown to the plaintext section of emails.
We made that decision many years ago and were kind of aware of some of the cracks.
@eviltrout / @codinghorror how do you feel about changing the mailer so it does HTML => text and uses that for the text part of the MIME vs using Post raw?
We now have a very robust patterns for doing HTML => text living inside Discourse and could make a custom one here. It is a pretty big job though cause we would have to be ultra careful with quotes and oneboxes.
For example now text contents has stuff like:
[quote="codinghorror, post:2, topic:112354"]
this is definitely a bug
[/quote]
This would probably be a nicer way of rendering this in a text email.
> this is definitely a bug
URL-TO-QUOTE