He's probably using List-unsubscribe. You can add headers to your emails that give users a way to unsubscribe from your list if they click "mark as spam" (and m

He's probably using List-unsubscribe. You can add headers to your emails that give users a way to unsubscribe from your list if they click "mark as spam" (and many do, despite an "unsubscribe" link appearing at the bottom of the email).