You've probably noticed the words "Mailer daemon" in
the header of an automatic e-mail reply or in a message that bounced back
from someone you tried to e-mail. A daemon (pronounced DEE-muhn or
DAY-muhn) is a program that runs in the background while a computer
performs other duties.
Daemons do certain tasks
automatically, at designated intervals, or in response to specific events.
This means that the user doesn't have to start the program. Daemons are common on computers
that are used as servers. A
mailer daemon that is installed on an e-mail system can respond to a piece
of incorrectly addressed e-mail by generating an automated message to the
sender telling them that the message was undeliverable.
The term "daemon" was inspired by the daemons of Greek mythology who were supernatural beings that ranked between gods and humans.
Source: The New York Times