You might like
[org-mime](http://orgmode.org/worg/org-contrib/org-mime.html) or
`mimedown`:

    ; For writing pretty mail
    (defun mimedown ()
      (interactive)
      (save-excursion
        (message-goto-body)
        (mml-unsecure-message)
        (let* ((sig-point (save-excursion (message-goto-signature) (forward-line -1) (point)))
               (orig-txt (buffer-substring-no-properties (point) sig-point)))
          (shell-command-on-region (point) sig-point "/usr/local/bin/pandoc --from markdown --to html --smart --standalone" nil t)
          (insert "