D.4 Confirmation before sending

To protect yourself from sending messages too hastily, you can add a final confirmation, which you can of course make as elaborate as you wish.

(add-hook 'message-send-hook
  (lambda ()
    (unless (yes-or-no-p "Sure you want to send this?")
      (signal 'quit nil))))

Another option is to simply set message-confirm-send to non-nil so the question “Send message?” is asked for confirmation.