Minimal syntax highlighting for emails. Colorize multiple quotes up to 5 levels in different colors. Use with thunderbird as external editor.

.emacs

;; http://ergoemacs.org/emacs/elisp_syntax_coloring.html
(setq fredmail-highlights
      '(("^[ \t]*>>>>>.*$" . 'org-level-5)
        ("^[ \t]*>>>>.*$" . 'org-level-4)
        ("^[ \t]*>>>.*$" . 'org-level-3)
        ("^[ \t]*>>.*$" . 'org-level-2)
        ("^[ \t]*>.*$" . 'org-level-1)
        ("\\*.*\\*" . 'markdown-bold-face)
        ))

(define-derived-mode fredmail-mode fundamental-mode "fredmail"
  "major mode for editing emails."
  (setq font-lock-defaults '(fredmail-highlights)))

(add-to-list 'auto-mode-alist '("\\.eml\\'" . fredmail-mode))