I write all my emails in plain text, not rich text or HTML, but I do use punctuation markup as has been popularized by Markdown and others (which were inspired by prior customs in plain-text email). There’s RFC 7763 now, which registers the text/markdown media type with mandatory charset and optional variant parameters. Can I change Apple Mail to always send a respective MIME header?

Content-Type: text/markdown; charset=UTF-8; variant=pandoc; extensions=-raw_html

I hope for some defaults write magic since there doesn’t seem to be a respective plugin available (yet). Something like this:

defaults write com.apple.mail UserHeaders \
  '{"Content-Type: text/markdown; charset=UTF-8; variant=pandoc; extensions=-raw_html"; }'