SPAM and Backscatter
SPAM, Backscatter, multiple MailerDaemon messages
The problem is quite complicated. We refer to it as "backscatter". This happens because your e-mail address has found its way onto a SPAM list. A spam list is a long list of e-mail addresses that spammers use to send bulk e-mail. Unfortunately, there isn't much you can do to decrease the number of such messages. The best step to take is to delete them. We may be able to help you set up a rule that will automatically move mailerdaemon messages into a separate folder, outside your inbox. Note that not all mailerdaemon messages are bad, there may be legitimate ones. See below for an explanation of how these messages get generated, and why there may be legitimate ones.
Almost every e-mail system has a "robotic" mailerdaemon account that automatically responds when an e-mail address listed in a message is NOT found (there's no person behind the mailerdaemon account). This mailerdaemon account has valid uses. Take this for example: Joe sends a message to an e-mail list (that consists of email@example.com, firstname.lastname@example.org and email@example.com). The message arrives in our (middlebury) mail system, and our (middlebury) mailerdamon finds that firstname.lastname@example.org doesn't exist, so it goes ahead and e-mails the other people on the list (including the sender, Joe) that email@example.com doesn't exist in our e-mail system. The message from mailerdaemon can help the sender to figure out why firstname.lastname@example.org doesn't exist (maybe the sender made a typo, and the correct address is email@example.com). Having some indication that the message did not reach firstname.lastname@example.org, is better than having no response and thinking that the message got there successful. That's how the mailerdaemon messages get generated, and why these messages may have valid uses.
So imagine the spammer sending a bulk e-mail message to a long list of people on different e-mail systems. If some of those people on the list don't exist, the mailerdaemon of each e-mail system will respond back, saying so. And since there's no person behind this account, there's no way to control these responses.