List-Unsubscribe

   The List-Unsubscribe field describes the command (preferably using
   mail) to directly unsubscribe the user (removing them from the list).

   Examples:

     List-Unsubscribe: <mailto:list@host.com?subject=unsubscribe>
     List-Unsubscribe: (Use this command to get off the list)
         <mailto:list-manager@host.com?body=unsubscribe%20list>
     List-Unsubscribe: <mailto:list-off@host.com>

     List-Unsubscribe: <http://www.host.com/list.cgi?cmd=unsub&lst=list>,
         <mailto:list-request@host.com?subject=unsubscribe>