The gist of it is that I’ve come up with an algorithm that works for all versions (I’ve seen in the wild).
Get the mailing list email address from either List-Post or X-BeenThere header
The mail of the sender will be in any of the following headers: From, Reply-To, X-MailFrom or X-Original-From. So iterate over those and return the first that doesn’t match the email address of the mailing list.