What is a mailer daemon – and why did my email bounce back?

It’s the dreaded bounce message: “Mail delivery failed: returning message to sender.” If you receive it, it means something went wrong and your email never made it to the intended recipient. But why can’t some emails be delivered, and who is the mysterious “mailer daemon” who sends them back to you?

mail.com explains these error messages and what you can do about them – and clears up the mystery of mailer daemon spam.

Email communication in cyberspace with envelope sign hologram over working cpu in background
Not sure why you received a bounce message? We explain how to fix a bounced email.

Why is it called a mailer daemon?

The name comes from Greek mythology, where a daemon was a benevolent but unseen spirit. In computer jargon, a “daemon” is simply an unattended software program that runs in the background. So the mailer daemon is the program that is responsible for delivering your emails – it also goes by the more humdrum name of “mail delivery subsystem.” 
If your email can’t be delivered by the mailer daemon, it automatically sends you an error message. Depending on the nature of the issue, this “bounce message” comes from your email server or the server of your intended recipient. In either case, you should check to see if it tells you why your email couldn’t be delivered. This is the first step in making sure your email will get through on the next try.

What do the error codes in bounced emails mean?

The mailer daemon message often contains an error code in the 400 or 500 range. The 400 codes generally represent a temporary problem. So usually all you need is a bit of patience – your message will be delivered after the issue sorts itself out, or you can wait a day and manually resend it. The 500 codes are another matter, unfortunately. The email cannot be delivered at all, so action may be required on your part, as explained below.

How do I fix a bounced email?

There are several reasons you may hear from the mailer daemon, including:

  1. The recipient’s address was incorrect
  2. The recipient’s mailbox was full
  3. The email server or system wasn’t working correctly
You can use the following checklist to figure out the source of the problem. We’ve provided a few examples of error codes and messages to help you, but please be aware that the exact wording may vary.

1. Is there a typo in the address or domain name?

A mistake in the recipient address is the most common reason that an email can’t be delivered. The error code will be in the 500 range, and the error message will be something along the lines of: “550 Requested action not taken: mailbox unavailable” or “Unknown user.” The mistake may also come after the @ sign, like if you accidently type “.com” instead of “.edu” in the domain name. Here your error message will read “Unrouteable domain” or “Host unknown.”

What you can do: Check the email address and domain name given in the error message. If you see a typo or mistake, try sending your email again with the correct address.

2. Is the recipient’s mailbox full?

If the person who you’re writing to already has a mailbox stuffed with messages, there is no room for yours. Your email is sent back to you with the error code 550: “Mailbox quota exceeded,” “User has too many messages on the server,” or “Permission denied. Command output: maildrop: maildir over quota.”

What you can do: Call your recipient or send them a very short email telling them to clean out their folders! ;-) Also, if you are trying to send files as attachments, make them smaller – send them as a ZIP file, for example. This compresses your files and packs them into a single folder, but the recipient can extract them and use them in their original size.
Pro tip: You don‘t need to send large attachments by email if you don’t want to. Instead, you can use our free Cloud to share files with others. Simply upload the file to you email account’s Cloud, select the file and right click, and select “Share.” A link will appear for you to send to others so they can view or download your file. This helps keep your emails small and lessens the risk of a bounce.

3. Is the email server or system available?

Sometimes the problem is that the email server of the intended recipient is not responding – maybe it is temporarily offline, for example. Then you might receive a message like “Could not send message for 4 hours.” If your error message reads “455 administrative reject” or similar, it’s a clear case of system overload and your email will not be going anywhere for a while. It will be delivered when the server has the capacity to process the request.

What you can do: This calls for patience: simply try again tomorrow. Read the message carefully, though – in many cases, it is simply a warning that there is a delay, and the system will keep trying to deliver your email automatically for several days. In that case, all you really can do is wait.

What if I get a bounce message for an email I didn’t send?

Not all mailer daemon notifications are harmless. Unfortunately, spammers and scammers have figured out how to send fake bounce messages and use them to spread contaminated links or phishing attacks.  So always be sure to ask yourself “Did I even send that email?” and read the subject line of the bounce message carefully. Often, you’ll spot right away that the purportedly bounced email address is nowhere close to any that you’ve written to lately. Never open or download an attachment from one of these messages – this might expose you to viruses or other malware.
Or if you receive a large quantity of mailer daemon messages, this could be a case of mailer daemon spam.

Wait. There is mailer daemon spam?

Maybe you have received mailer daemon spam before. It is noticeable when you look up and see (what feels like a gazillion) mailer-daemon failure bounce messages. You know it is spam because you did not send an email to these recipients, and you do not recognize the contents of these messages. And plus, it just looks real spammy.

We know you’re wondering “How is this happening to me? I am a real person.” Mailer daemon spam happens when spammers have used your email address to send spam mailings. Spammers will send a blast to email addresses in their database after having forged your email address as the sender. This does not necessarily mean that your email account has been compromised – they can fake the sender address without actually accessing your account. However, when the mailer daemon generates the error message for each email that was sent to an address that no longer exists, you will be the one that receives the bounce messages.

Unfortunately, mailer daemon spam can really be a sign that your computer is infected ­– for example, if you opened a phishing email containing malware – and the virus is sending messages to all your contacts. This can also generate mailer daemon bounce messages.

What should I do about mailer daemon spam?

Although mailer daemon spam does not always mean you have a malware or hacker problem, it’s better to be safe than sorry! If you are receiving mailer daemon spam, we strongly recommend you take the following steps:

1. Utilize antivirus scan and removal

It’s time to activate your computer’s antivirus protection by scanning and removing any viruses. You should scan all devices that you use to open your email. If a virus is detected, say good riddance, and quarantine or remove it as instructed by your antivirus software.

2. Change your password (to something more secure)

Immediately change your email password to a unique and secure alternative. This lets you make sure that no one has unauthorized access to your account so they can’t use it to send spam without your knowledge. You should only change your password once you have scanned your computer for viruses and removed any that were detected. If you change the password on a computer that is still infected with malware, the virus will automatically log it.

3. Get a hold of your contacts

It is important to notify your contacts that you were the victim of a mailer-daemon scam. Advise them about any suspicious activities initiated with your email address so that they can be wary and protect themselves as well.

We hope this post helped clear up some of the mystery surrounding the mailer daemon. We look forward to your feedback below!

Images: 1&1/GettyImages

This article first appeared on Sept. 13, 2021 and was updated on Oct. 27, 2022

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