This article https://begriffs.com/posts/2018-09-18-dmarc-mailing-list.html suggests that mailing lists should avoid modifying the subject and body of messages. If either is modified, the DKIM check will fail. Because SPF usually cannot work with mailing lists (return path is set to the list), it means both tests fail, and DMARC checks fail, too. I guess most mailing lists, including many Mozilla mailing lists, are still configured to modify the subject (insert a mailing list identified), and modify the body (append a footer with links to list info and list unsubscribe). If we want to encourage mailing list administrator to stop these modifications, we'd have to convince them they are unnecessary. In order to be considered unnecessary, mail clients should discover the List-Id and List-Unsubscribe headers, and adjust the displayed UI accordingly. Requests: (a) If a message contains a List-Id: header, show the list identifier somewhere in the message header display, close to the From area. (b) If a message contains a List-Unsubscribe header, offer a button or menu item that allows the user to unsubscribe from the list.