Software in a mail server that delivers messages to recipients. When you get a MAILER-DAEMON@whatevercompany.com message in your inbox, the server at that company is informing you that it is returning your message because of some failure. The "to" e-mail address may no longer be valid, or there may be a problem routing the message to the appropriate mail server. Your domain name may be on a blacklist, and the server is refusing all incoming messages from it. See blacklist.

Chances are also good that you never sent the message in the first place. Your e-mail address could have easily been copied by a worm from someone else's address book and used as a "from" address without your knowledge. See daemon and e-mail virus.