(setf (cadr (assoc "message/rfc822" mm-inline-media-tests))
      'mm-inline-message-bracketed)

(defun mm-inline-message-bracketed (handle)
  (insert ",---\n")
  (let ((end (point-marker)))
    (set-marker-insertion-type end t)
    (mm-inline-message handle)
    (while (< (point) end)
      (insert "| ")
      (beginning-of-line 2))
    (move-marker end nil)
    (insert "`---\n")))