List-Unsubscribe header
From Just Solve the File Format Problem
The List-Unsubscribe header is a header for e-mail messages that indicates how to unsubscribe from the list that the message was sent under. It is intended to be used in bulk mailings. The contents are a URL that can be used to remove yourself from the list, either a web link or a mailto: URL with an e-mail address you can send to in order to remove yourself from the list.
