If Postbox detects a “List-Unsubscribe” attribute in a message header, it provides you with an Unsubscribe button in order to make it easy for you to stop receiving messages from the message's sender, should you wish to do so.
Click Unsubscribe to go directly to the subscription list's Unsubscribe page, or to create the required Unsubscribe e-mail to send to the service.