Glossary · Deliverability
One-click unsubscribe (RFC 8058) lets a mail app unsubscribe a user with a single background request, now required for bulk marketing mail to Gmail and Yahoo.
The message carries an HTTPS URL in List-Unsubscribe plus List-Unsubscribe-Post: List-Unsubscribe=One-Click. When the user taps unsubscribe, the mail app sends a POST to that URL: no login, no confirmation page.
Both headers must be covered by the message's DKIM signature, and the unsubscribe must take effect promptly; Google and Yahoo ask for within two days. It's required for marketing mail from bulk senders, not for transactional or one-to-one mail.
List-Unsubscribe: <https://acme.com/u/8f3a>
List-Unsubscribe-Post: List-Unsubscribe=One-ClickEvery domain you run in one inbox, from $3 a mailbox. 14 days free, no card.