Side-note: for list-unsubscribe, do you determine the subscriber's identity that needs to be unsubscribed based on the sender or the receiver (like <guid>@unsubscribe.service.com)?
Reason I'm asking is Unsubscribe rarely works for me due to my catch-all not SENDING emails from the address it was received on. It sends it from my actual address. Very annoying.
The RFC 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.
Reason I'm asking is Unsubscribe rarely works for me due to my catch-all not SENDING emails from the address it was received on. It sends it from my actual address. Very annoying.