I believe that the mail back feature is to prevent someone from being joined to a list without their consent which would fill up their mailbox with unwanted mail. It's an old trick. There used to be hacker programs that you could use to join someone to 60 or more lists at one time. Spam seems to be on the rise. I've recently upgraded my mail server to take advantage of the anti-Spam features. I've spent a lot of time analyzing Spam. The overwhelming majority of Spam originates here but bounces off servers overseas. Denying mail from Japan has cut down my Spam by about 70%. I'm currently working on the other countries that are Spam sympathizers.
