Moin,
the mails from Discourse groups are missing the hint, that they contain Markdown. So my mail client shows me only the plain text, without rendering the as markdown. The simple solution would be to extend the content type from Content-Type: text/plain; charset=UTF-8
to Content-Type: text/plain; charset=UTF-8; markup=markdown
For testing: The mail client I use is MailMate. For Thunderbird there is an MD addon (Markdown-here). AirMail can use MD and I don’t know any others at the moment.