Addendum: if you host a mailing list server such as Mailman, you'll need to do one of (1) don't change the content or DKIM-signed headers at all (e.g. by adding

Addendum: if you host a mailing list server such as Mailman, you'll need to do one of (1) don't change the content or DKIM-signed headers at all (e.g. by adding Subject tags or unsubscribe instructions); (2) rewrite the From: header so that you don't look like you are impersonating the individual senders; (3) use ARC [https://en.wikipedia.org/wiki/Authenticated_Received_Chain https://en.wikipedia.org/wiki/Authenticated_Received_Chain] signatures. I don't know if Gmail and the other big providers actually honor ARC headers yet, but if they ever do, that would seem to be the cleanest solution if you need to add unsubscribe instructions or similar banners.