system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do they call the deliverer of unsent email the "mailer-daemon" ? What is it's origin? Is it just a name for some type of protocol? Or something else?

I'm talking about the auto reply emails that bounce back to you when sending emails to an address that no longer exists.
gpt
A "Daemon" is computer program that runs as a background process. The Mailer-Deamon is a program that takes incoming mail and moves it to the recipient's folder. If a recipient cannot be found, the mailer-daemon send back a notice that the address is incorrect.    
     
The term Daemon comes from Maxwell's demon, a hypothetical being from a thought-experiment by the famous physicist. Daemons are mostly a term used on unix (linux, solaris, AIX, etc) systems - windows uses the term "Services".