What is a mailer daemon?

Despite its rather sinister name, a mailer daemon is a vital component of email delivery – the bearer of bad news when a message can’t be delivered

a demon

Tuesday, 13 October, 2020

If you receive an email from an account titled Mailer Daemon, it may not be immediately obvious where the message has come from, or how it got your address.

What you’ve received is an automated notification from the software responsible for delivering email messages to specific user addresses.

A mailer daemon performs an important job, and when it gets in touch, you should really pay attention.

The devil’s in the detail

The term ‘mailer daemon’ dates back to the 1960s, and the Massachusetts Institute of Technology.

Named after the Greek term for a general spirit (rather than an evil entity), the word daemon was chosen by MIT to describe a background process which happens out of sight.

In terms of the new-fangled electronic mail being developed at the time, it described software which managed undeliverable messages.

If someone sent a message to an incorrect username at a valid network, mailer daemon software would reply with an automated error message.

And over half a century later, daemons continue to perform the same job.

When that unmistakeable term pops up in your inbox, it almost always indicates a problem with a message that’s recently been sent.

This could be for a variety of reasons:

  1. A message you’ve sent is undeliverable because the named recipient’s email account doesn’t exist, or a formerly active account has been closed.
  2. Message delivery has been unexpectedly delayed, possibly due to network congestion or a full mailbox. Remarkably, this 1990s phenomenon does still happen.
  3. Your email account or device has been corrupted by a worm or virus, which has been indiscriminately sending out junk mail without you noticing.
  4. The message can’t be routed to the appropriate mail server due to technical problems. In this instance, the daemon will attempt redelivery multiple times before giving up.
  5. The recipient’s email server has refused to accept a message from your account, probably because your email address has ended up on a blacklist.

This latter scenario requires immediate attention, because if one mailbox thinks you’re a spammer, others probably will, too.

Organisations like Spamhaus maintain blacklists of dubious or spammy email accounts. Getting yourself removed from one of these lists is tricky, but not impossible.

It’s rare to find out why a legitimate email account has been blacklisted, though it can be as simple as a handful of recipients marking your emails as spam.

Add a ‘please unsubscribe’ suffix to your email signature, encouraging people to request that you delete their details instead of hitting the ‘spam’ button.

Finally, be aware that daemon spam also exists.

Bogus messages urge you to click a link to a compromised website that will infect your device with malware or spyware.

Don’t automatically assume a daemon notification is truthful, and always check whether the message it claims to be reporting was actually sent.

Neil Cumins author picture


Neil is our resident tech expert. He's written guides on loads of broadband head-scratchers and is determined to solve all your technology problems!