RFC 8058 (one-click unsubscribe)
Also known as: List-Unsubscribe-Post, One-click unsubscribe
Definition
RFC 8058 defines the List-Unsubscribe-Post header that lets a mail client unsubscribe a recipient with a single click, and Gmail and Yahoo require it from anyone sending 5,000 or more messages per day.
Why it matters
Implementing it means sending both List-Unsubscribe and List-Unsubscribe-Post headers, and exposing an endpoint that accepts a POST and suppresses the address without further confirmation.
A confirmation page defeats the purpose and violates the specification: the POST itself is the unsubscribe.