List-Unsubscribe is an email header, defined in RFC 2369, that tells mailbox providers how a recipient can unsubscribe. It isn’t visible in the email body. Instead, providers such as Gmail, Yahoo and Apple Mail read it and may show their own “Unsubscribe” option near the sender name, giving people a consistent way to leave without searching the footer.
The header can contain a mailto address, an HTTPS link, or both. With mailto, the mail app sends an email to that address and your system must process it. With HTTPS, the app visits the link. On its own, an HTTPS link might open a web page asking the person to confirm, which is why the newer one-click standard adds a second header, List-Unsubscribe-Post, so the provider can complete the unsubscribe in the background without opening anything.
This header helps senders as much as recipients. When people find it easy to unsubscribe, fewer of them use the spam button, and spam complaints hurt your reputation far more than unsubscribes do. That is why Gmail and Yahoo require bulk senders to support one-click unsubscribe through these headers for marketing and subscribed messages.
Providers decide when to display the option, often based on your reputation and volume, so you may not see it on every email. To check it’s present, open a sent email and view the original message or headers. The header must also be covered by your DKIM signature for one-click to be accepted.
A subscriber in Indore gets a weekly deals email she no longer wants. Next to the sender name in Gmail she taps “Unsubscribe”, confirms in Gmail’s pop-up, and is removed without leaving the app. The brand loses one subscriber instead of receiving a spam complaint.
Every MailNesto campaign includes List-Unsubscribe and List-Unsubscribe-Post headers, so mail apps that support one-click unsubscribe can remove someone instantly, and the change is logged in their consent history.