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?
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Not sure why you received a bounce message? mail.com explains how to fix a bounced email.
mail.com sheds some light on these error messages and what you can do about them.

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.

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.

mail.com 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 your 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.

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

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