Overview ---------------------------------------- There was a [recent announcement](https://blog.google/products/gmail/gmail-security-authentication-spam-protection/) that Gmail would begin [requiring `List-Unsubscribe=One-Click`](https://support.google.com/mail/answer/81126) from mailing lists circa Feb 2024. The relevant protocols are described by: * https://datatracker.ietf.org/doc/html/rfc2369 * https://datatracker.ietf.org/doc/html/rfc8058 Example use-case ---------------------------------------- 1. Create a "New Mailing" 2. Send it 3. As a recipient, view the headers of the message 4. Check the content of `List-Unsubscribe` and `List-Unsubscribe-Post` Current behavior ---------------------------------------- CiviMail generates one header: ``` List-Unsubscribe: <mailto:u.1.1.rrpjscmw7whtdg4e@example.org> ``` Proposed behavior ---------------------------------------- CiviMail generates two headers: ``` List-Unsubscribe-Post: List-Unsubscribe=One-Click List-Unsubscribe: <https://example.org/civicrm/mailing/oneclick?jid=1&qid=1&h=rrpjscmw7whtdg4e> ```