RFC 8058 defines a way for a sender to advertise an HTTPS one-click unsubscribe action.

The message contains both:

List-Unsubscribe: <https://example.com/unsubscribe/opaque-token>
List-Unsubscribe-Post: List-Unsubscribe=One-Click

The receiving system can perform an HTTPS POST without sending the user through a confirmation page.

It is not just a body link

A visible unsubscribe link in the message body is useful, but it is not the same protocol mechanism as RFC 8058 one-click unsubscribe.

The specification also requires a valid DKIM signature covering the relevant list-unsubscribe headers.

Current provider requirements

Large mailbox providers can impose additional requirements on bulk or subscription mail. Google, for example, requires one-click unsubscribe for applicable marketing/subscribed traffic under its sender rules. Always verify current provider guidance because those requirements evolve.

Reference material