Glossary · Deliverability
List-Unsubscribe is an email header giving a mailto: or web address for unsubscribing, which mail apps turn into a built-in unsubscribe button.
Defined in RFC 2369, the header lets Gmail, Apple Mail, Outlook and others show their own unsubscribe link next to the sender, so people leave your list instead of reporting you as spam.
On its own the header may open an email or a web page. Bulk senders to Gmail and Yahoo now also need one-click unsubscribe, which adds a second header so the mail app can unsubscribe in the background.
List-Unsubscribe: <https://acme.com/u/8f3a>, <mailto:unsubscribe@acme.com?subject=8f3a>Every domain you run in one inbox, from $3 a mailbox. 14 days free, no card.