It’s a friendly helper.
Some might argue there are definitely demons at play in various aspects of the internet (certain aspects of social media come to mind), but this isn’t about that.
A daemon (spelled with that “a”) is different than a demon, even though they’re typically pronounced the same way.
And daemons are a fundamental part of how modern operating systems, and indeed the internet, work today.
Mailer-daemon
Mailer-daemon isn’t a demon; it’s a helpful background program that keeps email running. When something goes wrong with a message you sent, mailer-daemon sends you a note explaining the problem. You’ll see it most often on bounce messages. Pay attention to what mailer-daemon tells you.
137 daemons in a box
“Daemon” is another name for “background service”. The term is evidently a nod to Maxwell’s Demon, part of an 1867 thought experiment relating to the second law of thermodynamics. The demon was a “being who can play a game of skill with [..] molecules”.
In today’s parlance, a daemon is software performing behind-the-scenes tasks.
In Windows, we call them services, and there are many. As I type this on my desktop PC, there are approximately 137 processes running now.
In comparison, the Linux server from which you’re likely reading this lists 27 services.
Regardless of operating system, they could just as easily be called daemons. In Linux, you’ll see both terms used — service and daemon. The tide is slowly changing to service, but older processes may still be referred to as daemon.
And email is most definitely an old process — older even than the internet itself.
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Mailer-daemon
The concept of an always-running program to process email dates back to the days of the internet’s predecessor, ARPANET.
Software running on a network-connected machine awaits incoming email messages to be processed and passed on to their destinations. It typically has no real user interface, other than a configuration file telling it what to do and a log file listing what it did. It just runs in the background performing its task.
It’s a service. Or daemon. A mailer-daemon is any software running in the background that processes email.
Mail from mailer-daemon
One of the things that mailer-daemons handle is error- and status-reporting. Examples include another mail server that’s not working, misdirected email messages, refusal to accept a particular message, notification that a particular message has been delivered, and so on.
Many, if not most, of those cases involve sending an email message to the original sender informing them of the problem or sharing status information.
Depending on the specific mail software being used, that message can come “from” many different names. One of the most common is “mailer-daemon” — specifically, an email address like “mailer-daemon@somerandomservice.com”, where “somerandomservice.com” is replaced with the actual email service involved.
In practice, you’ll probably see mailer-daemon most often for email bounce messages. Be sure to read the message for the details of what’s being reported.
Do this
Mailer-daemon itself is nothing to be concerned about. It’s a fundamental part of how email works. Pay attention to the messages it sends you.
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