"mailer daemon" is a just common name for the main persistent background process of mail server that manage emails.

"Daemon" is a program that runs in background and have no user interface input or output. Only other programs could interact with daemons, usually via network, locally or remotely.

So, in your metaphor "mailer daemon" is a Lucifer in a cage where it is isolated from humans and forced to put stamps on emails and sort them. :)