List-Unsubscribe / one-click unsubscribe

List-Unsubscribe is a message header advertising how recipients can unsubscribe without hunting through the message body; RFC 8058's one-click variant (List-Unsubscribe-Post) lets mail clients offer a single-click opt-out that senders must honor promptly. One-click unsubscribe is now a bulk-sender requirement at major providers, and honoring it within days is expected.