http://www.rfc-editor.org/rfc/rfc8058.txt
RFC 8058: Signaling One-Click Functionality for List Email Headers, J. Levine, T. Herkula2017-01-01T23:00:00-00:00This document describes a method for signaling a one-click function for the List-Unsubscribe email header field. The need for this arises out of the actuality that mail software sometimes fetches URLs in mail header fields, and thereby accidentally triggers unsubscriptions in the case of the List-Unsubscribe header field.