small program in Unix that check for activity
Returns bounced mail. Makes sure that programs are supposed to run at certain times. Many mailer daemons tell exactly what the problem is. If the message starts with 550, it is an error message telling the user what went wrong with delivery.
A program associated with UNIX systems that performs a housekeeping or maintenance utility function without being called by the user. A daemon sits in the background and is activated only when needed, for example, to correct an error from which another program cannot recover.