Recently the standard has been upgraded to resolve an issue with the unsubscribe URL link method. Anti-spam software frequently checks every link in an email message for malicious content. This resulted in a lot of accidental unsubscribes. The new standard outlined in RFC 8058 [link to https://tools.ietf.org/html/rfc8058 ] adds another line that prevents accidental unsubscribes by anti-spam software while retaining the hands-off one click unsubscribe functionality. It looks like this:
List-Unsubscribe: <mailto:email@example.com?subject=unsubscribe> <http://www.acme.com/unsubscribe.html?opaque=987654321>
Now if the link is manually followed in a browser or by anti-spam software, a landing page will be displayed that does not result in an immediate unsubscribe but would offer an unsub button instead. The email client will perform a POST request on this URI and sending the key/pair value from this header will create the unsubscribe without any user interaction.
The most popular email clients all support the Mailto Method and support for the RFC 8058 is gaining ground. Here’s a chart showing which email client supports which method.