The RFC https://www.ietf.org/rfc/rfc2369.txt https://www.ietf.org/rfc/rfc2369.txt Section 3.2 is not specific on this - but the examples only show the To addres

The RFC https://www.ietf.org/rfc/rfc2369.txt https://www.ietf.org/rfc/rfc2369.txt Section 3.2 is not specific on this - but the examples only show the To address, and no unique identifier beyond that, so it might not work out well for you for mailto list-unsubscribes. It also prefers mailto over https. If we build this as a mandatory feature at MailPace, we'll use an HTTPS webhook with a unique identifier for the email, so if you unsubscribe from a list sent via us, it will work for you.