I write all my emails in plain text, not rich text or HTML, but I do use [punctuation markup][LML] 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`][7764] 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][MDhere]). Something like this:

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

  [LML]: https://en.wikipedia.org/wiki/Lightweight_markup_language
  [RFC]: https://www.rfc-editor.org/rfc/rfc7763 "RFC7763"
  [7764]: https://www.rfc-editor.org/rfc/rfc7764 "RFC7764. currently registered variant identifiers: MultiMarkdown, GFM, pandoc, Fountain, CommonMark, kramdown-rfc2629, rfc7328, Extra"
  [MDhere]: https://github.com/adam-p/markdown-here/issues/80