If you've ever mistyped an email address, you have surely been paid a visit by the Mailer-Daemon (in the form of a reply email). Who is this terrifying ghoul and why did it journey from the fiery depths of hell through your computer to remind you of your mistake?
Simply put, a daemon is a computer program that automatically performs a task. This seems unimpressive, but there was a time when computers relied on human beings to input commands or information in order to work. A daemon takes simple errands off an operating system's plate, freeing it up to handle more taxing things. Daemons are your friends, and they don't come from hell.
A daemon operates out of sight from the users' eyes, and it is this quality that inspired its name. Members of MIT's Project MAC are credited with coming up with the term during their early research on artificial intelligence and computing processes (the group was created in 1963). According to Project MAC's Fernando J. Corbato, the term for this new type of computing was inspired by Maxwell's daemon of physics and thermodynamics. "Maxwell's daemon was an imaginary agent which helped sort molecules of different speeds and worked tirelessly in the background," he told the Austin Chronicle. "We fancifully began to use the word daemon to describe background processes which worked tirelessly to perform system chores." The daemon spelling comes from the Greek use of the word, and it refers to a general spirit and not necessarily an evil being.
(For a more in-depth look at Maxwell's demon, Khan Academy has a rundown.)
When email was first being developed, programmers implemented these daemons into the software in order to automatically alert users if they entered an address wrong or made a formatting mistake. The name "Mailer-Daemon" stuck, and that's why we still see it today, materializing in our inboxes from the mysterious beyond.