What is a MAILER-DAEMON failure notice?

MAILER-DAEMON is 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. Most likely the "to" e-mail address may no longer be valid, or was entered incorrectly. There may be a problem somewhere in cyberspace 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.