One-click unsubscribe lets a reader opt out with a single click on a button their email app shows next to the sender's name, without opening a link or visiting a page. It works through two email headers.
The headers
List-Unsubscribe: where an opt-out request goes, an HTTPS address, amailto:address, or both.List-Unsubscribe-Post: List-Unsubscribe=One-Click: tells the app it may send the opt-out straight away (the RFC 8058 standard).
Who requires it
Since 2024, Gmail and Yahoo require one-click unsubscribe from bulk senders of marketing email, with opt-outs honoured within two days.
Why cold emailers use it
A reader who wants out will press something. Given an easy Unsubscribe button, many press that instead of "Report spam", which protects your complaint rate and your domain. mailzy adds the header to campaign emails by default and stops all email to anyone who uses it. Build the headers for your own sending with the free list unsubscribe header generator.