Guardians of routine: A brief history of MAILER and other DAEMONS

If you’ve ever sent a message to an old email address and gotten a bounce-back reply, it often comes from a daemon, specifically a MAILER-DAEMON. 

Why a daemon? And why would such a creature be spending time sending us these helpful yet mundane messages that our communication wasn’t received? 

For decades I have gotten these messages but never looked for the answer, until today. I got one of these auto-replies at the same time that I was reading Neal Stephenson’s book Snow Crash, and the concept gained new meaning for me…

The name “daemon” comes from Greek mythology, where a daemon was a benevolent but unseen “guardian spirit.” Daemons act as intermediaries between the mortal world and the divine. In computer jargon, a “daemon” is an unattended software program that runs in the background. Pronounced “dee-mun,” it is a Unix program that executes an operation when required, functioning like an extension of the operating system. 

Daemons in computing are named after a thought experiment from the 1800s that would hypothetically violate the second law of thermodynamics (i.e. energy cannot transfer from a body at a lower temperature to a body at a higher temperature without the addition of energy). Proposed by the physicist James Clerk Maxwell in 1867, he described an entity that could “play a game of skill with the molecules,” later dubbed a “demon.” In his experiment, a demon controls a small massless door between two chambers of gas. As individual gas molecules approach the door, the demon quickly opens and closes the door to allow fast-moving molecules to pass through in one direction and slow-moving molecules to pass through in the other. Because the kinetic temperature of a gas depends on the velocities of its molecules, the demon’s actions would cause one chamber to warm up and the other to cool, a physical impossibility.

A century later in computers, the concept of a “demon” in a system evolved into print spoolers, e-mail handlers, or a schedulers that start up their process at a designated time. The choice of the term “daemon” for processes like “mailer-daemon” reflects the idea of an autonomous, unseen entity performing tasks on behalf of users without requiring their direct involvement. This aligns with the concept of background processes in computing, which operate independently to manage various tasks and services. The term “mailer-daemon” specifically refers to an automated system that generates email delivery status notifications. I envisioned a colorful troupe of daemons hanging around the mailroom, waiting to be called into action! 

In Neal Stephenson’s novel Snow Crash, daemons are described as “low-level utility software” and a “fundamental part of the operating system.” In his metaverse, a daemon is like an avatar but doesn’t represent a human being. As personified pieces of software or robots, they handle routine activities, allowing users to focus on other things. The concept of daemons is an important part of the virtual reality and technology-driven world depicted in the book. In his story, some of the daemons run errands, serve imaginary drinks, and even act as bouncers for virtual metaverse clubs, tasks much more interesting than hanging around the mailroom. 

So the next time you get that bounced-back email message, just imagine that benevolent piece of software in the form of a kind daemon on the other side, working for you so you can focus on more important things.