List-Unsubscribe header
The List-Unsubscribe header is an email header, originally defined in RFC 2369 and extended by RFC 8058's one-click mechanism, that lets a sender publish a mailto or HTTPS link a mail client can use to unsubscribe a recipient without them ever opening the message or hunting for a link in the body. RFC 8058 adds a paired List-Unsubscribe-Post header so a client can fire a one-click POST request instead of just opening a mailto link, which is now effectively required by Gmail and Yahoo's bulk-sender rules for anyone sending high volumes of mail. Omitting it, or publishing it but letting the link fail, pushes frustrated recipients toward the spam-complaint button instead, and complaint rate is one of the reputation signals that feeds into whether a sending domain or IP ends up on a blocklist. AdminKit's blacklist check reports that downstream outcome, even though it doesn't parse the header itself.
By AdminKit team, Netbase JSC · Updated