Home » Email Marketing » Understanding Mailer Daemon: How to Manage Bounce-Back Emails

Understanding Mailer Daemon: How to Manage Bounce-Back Emails

I think most of us have seen it in our inboxes at least once. You send an email to what you thought was an active recipient, and then you get it: the mailer daemon.

You double-check that you spelled the email address correctly and see that you did, but the email bounces back again.

What in the world is going on?

Mailer daemons can be confounding if this is your first experience with them. Besides the obvious confusion they cause, they can also negatively affect your marketing and sales goals, hampering your email marketing and limiting who you can communicate with.

That’s why understanding mailer daemon messages and why you receive them is so critical for business email communication. This guide will demystify these messages and provide actionable advice for small business owners.


Key Takeaways for Small Business Owners

  • Promptly addressing mailer daemon messages to maintain email deliverability and sender reputation is crucial.
  • Proper email list management can significantly reduce occurrences of mailer daemon errors.
  • Clear, concise email communications will prevent email bounce-backs.

What Is a Mailer Daemon?

First, we need some exposition. A mailer daemon message bounces back immediately when an email failure occurs. Why the daemon name? It’s not quite “demon,” but it certainly conjures that word. So, is that what it means?

Not exactly. The term is named after a Greek mythological figure called the daemon. This spirit wasn’t quite a demon, as it was known for being benevolent and never seen.

The latter attribute is likely why daemons have been adopted into computer and tech terminology. After all, a daemon refers to software left running unattended while you’re doing other tasks, while a mail daemon specifically delivers emails … or doesn’t, which is when you get an error message.

Email daemon notifications come your way via Simple Mail Transfer Protocol or SMTP. That explains why they arrive so quickly.

By the way, daemon messages are sometimes referred to as Delivery Status Notifications or DSNs, Non-Delivery Notifications or NDNs, or Non-Delivery Reports or NDRs.

Common Types of Mailer Daemon Messages

Ultimately, mailer daemon messages break down into one of two types of bounce-backs: hard or soft. Let’s explain.

Soft bounce-back messages

A soft bounce mailer daemon email refers to what’s usually a temporary delivery problem. Here are the reasons you will receive a soft bounce:

  1. The message cannot be sent for unknown reasons.
  2. The email can’t be relayed between two email servers.
  3. The email doesn’t pass the sender requirements for the recipient server.
  4. The email doesn’t pass the antivirus requirements for the recipient server.
  5. The email doesn’t pass the anti-spam requirements for the recipient server.
  6. The email doesn’t pass the DMARC authentication requirements for the recipient server.
  7. The email contains content that’s been blocked.
  8. The email was sent to a domain name that doesn’t exist.
  9. The email is too long.
  10. The recipient’s mail server has received too many emails in a short period.
  11. The recipient’s mailbox is inactive.
  12. The recipient’s mailbox has not yet been correctly configured.
  13. The recipient’s mailbox is full.

Soft bounces can usually be resolved through time and a bit of patience, so if you receive one of these mailer daemon messages, wait until later or even the next day, then try sending the email again.

Hard bounce-back messages

If the mailer daemon hard bounces, the error is permanent. Whether you wait an hour or a month, you’ll still get a bounce-back each time you try to contact the recipient.

The recipient’s server might block all incoming emails or perhaps only your messages. If the recipient’s email address no longer exists, you’ll also get a hard bounce for your troubles.

Read also: Learn To Manage Email Bounce Codes in 10 Minutes

Common Causes of Mailer Daemon Errors

Okay, so you received a mailer daemon error, but what does it mean? And what caused it?

When an SMTP server generates a mailer daemon message, it will include an accompanying error code. The code will have three digits, and you’ll see either a 1, 2, 3, 4, 5, 6, or 7.

A 4 represents temporary or soft bounces, and 5 refers to a fatal or permanent error.

Here’s a rundown of what each error means presented in handy table format.

1 Codes

X.1.0 The status for other addresses
X.1.1 The mailbox address has a bad destination
X.1.3 The mailbox address syntax has a bad destination
X.1.4 The destination mailbox has an ambiguous address
X.1.5 The destination mailbox address is valid
X.1.6 The destination mailbox has moved
X.1.7 The sender’s mailbox address syntax is bad
X.1.8 The sender’s address system is bad


2 Codes

X.2.0 The status for undefined or other mailboxes
X.2.1 The mailbox has been disabled and cannot accept new messages
X.2.2 The mailbox is full
X.2.3 The length of the email surpasses the administrative limit
X.2.4 A problem occurred with the mailing list expansion


3 Codes

X.3.0 The status for undefined or other mailboxes
X.3.1 The mail system is full
X.3.2 The system does not accept network messages
X.3.3 The system cannot use the selected features
X.3.4 The size of the message exceeds the system allowance


4 Codes

X.4.0 The status for undefined or other mailboxes
X.4.1 The system didn’t receive an answer from the host
X.4.2 The connection is bad
X.4.3 A routing server failure occurred
X.4.4 The server cannot route the email
X.4.5 Network congestion occurred
X.4.6 The system detected a networking loop
X.4.7 The delivery timeframe expired


5 Codes

X.5.0 The status for undefined or other mailboxes
X.5.1 The command is invalid
X.5.2 A syntax error occurred
X.5.3 The email has excess recipients
X.5.4 The command arguments are invalid
X.5.5 The protocol version is incorrect


6 Codes

X.6.0 The status for undefined or other mailboxes
X.6.1 The type of media sent isn’t supported
X.6.2 A conversion is required but is prohibited
X.6.3 A conversion is required but is unsupported
X.6.4 A conversion occurred, but with a loss
X.6.5 A conversion failed


7 Codes

X.7.0 The status for undefined or other mailboxes
X.7.1 The message was refused, and the delivery was unauthorized
X.7.2 Expanding the mailing list is prohibited
X.7.3 A security conversion is required but cannot occur
X.7.4 The security features are unsupported
X.7.5 A cryptographic failure occurred
X.7.6 The cryptographic algorithm is unsupported
X.7.7 The message integrity failed


Now, let me explain more simply why you might receive an error daemon message after sending a business email.

Incorrect email address

Listen, typos happen all the time. Unlike when working in Google Docs or Microsoft Word, most of the time, there is no red squiggle underneath a mistyped email address. If you missed a letter or added a period two spots over when it should have only been one, you will get a mailer daemon email error unless and until you correct the offending address.

Server issues on the recipient’s end

Sometimes, the reason you see a mailer daemon email in your inbox has less to do with what you did and more to do with the recipient’s server.

There could be one or several issues at play preventing the recipient from getting your email, and depending on what they are and their extent, you will be met with either a soft or a hard bounce.

Any type of connectivity issue, interruption, network congestion, or failed authentication can stop your message dead in its tracks.

Email blocked by spam filters

Much more critically, the recipient’s server might associate your email address with spam, thus they block the message from going through. Your email is stuck in a spam filter, where it will sit forevermore until the recipient either stumbles upon it and moves it out of spam or it’s permanently deleted from their inbox after 30 or 60 days.

So, why in the world are you being reported for spam? I’ve talked about email spam rules extensively, but this seems like a good time for a refresher.

High complaint rate: Every email service allows users to report a message as spam. Did you know internet service providers or ISPs keep track of how many times an email address is reported as spam? This is known as your complaint rate, and when it’s high enough, ISPs can block you from sending messages.

High bounce rate: ISPs also pay attention to your email bounce rate. If most of your emails to various recipients bounce, whether a hard or soft bounce, the ISP might put the kibosh on any future email activity.

High email volume: Are there times when you need to mass-email 4,000 people? Sure, I’d imagine there are. However, if you do that too frequently, especially without warming up your IP first (aka slowly sending an increasingly higher volume of emails), the ISP will think it’s spam.

Spammy email content: If you use spam words, send emails with misspelled words, capitalize almost everything in the message, play around too much with text color, and use a lot of exclamation points and dollar signs, that tells an ISP your message is spam, so it will be treated as such.

Strange email address: If you regularly communicate with your audience using one email address, then suddenly switch to another one, that can be all it takes for your message to be identified as potential spam.

Your IP is blocked: Your IP address could be blocked for bulk emailing or violating acceptable usage policy rules. Sending content laden with viruses, including attachments, is another reason for IP blocking.

Your email doesn’t have a subject line: A lack of a subject line in your email will immediately tip off ISPs, who will more than likely block your message from going through to the intended recipient.

You send unapproved content: Emailing unapproved HTML links is an excellent way to ensure you can’t reach the recipient with this and future emails. The ISP may block you on phishing URL suspicion.

Your email has a virus: Whether accidentally or intentionally, sending viruses, malware, trojans, and other malicious content through links or attachments will result in your email being blocked for spam, and rightfully so!

You added too many images: Overdoing it on the images can hint to an ISP that your behavior might be spammy, so go easy there.

You send spam traps: Honeypots or spam traps exist to collect spam, and they will get stopped by ISPs sooner or later. Don’t send them or associate with them.

Poor sender reputation: All email addresses have a sender reputation, which is dictated by the above factors, including your email unsubscribe rate, send frequency, domain reputation, and engagement levels. If yours is poor, you may see a lot of mailer daemon messages.

Attachment size exceeds the limits

Besides the above, watch your attachment file sizes. While the limit varies by email client, you’re typically capped at 20-50 MB for attachments.

How To Respond to Mailer Daemon Messages

Mailer daemon example
Image courtesy of Mailmodo

Your business relies on email to streamline the flow of communication, as the world sends 347.3 billion emails a day, with you contributing to that tally.

So, what do you do when you receive a mailer daemon email in response to a message you just sent?

Let’s break it down.

Step 1 – Open the email

Daemon messages will include an attachment with a breakdown of the error code. Open the attachment to see the code.

Step 2 – Review the error code

You don’t have to scratch your head about mailer daemon email codes since I demystified them above. Determine what yours means, as that will inform you on how to proceed.

Step 3 – Fix the error (If possible)

If the issue was on your part, such as a misspelled username, fix it. You can also get in touch with the recipient by phone if you’re familiar with them and ask if they’re having server issues or if you did something that prevented the email from going through.

You should also reduce your spam practices. As your sender reputation improves and you get added to more whitelists instead of blacklists, you should have fewer difficulties sending emails.

Step 4 – Resend the email

Once you believe you’ve fixed the issue, try sending the email again. It should go through unless the issue isn’t on your part or you get a hard bounce.

Preventing Mailer Daemon Errors

You shouldn’t have to jump through hoops to get your messages sent. The following strategies will help your messages go through the first time.

Verify your email address before sending

Your small business likely has several email addresses you use to communicate with customers, such as auto-confirmation and auto-shipping emails, sales emails, and marketing emails. That’s okay.

However, before you begin communicating with your audience using an email address that’s unfamiliar to them, ensure you verify your email. You can use tools like ZeroBounce, NeverBounce, EmailListVerify, and more for this job.

Adhere to email send limits

Email send limits exist for a reason. When you violate them, whether through frequency or file size, you run the risk of incurring the wrath of an ISP. Eventually, your emails can stop going through, which isn’t worth the risk.

Learn the send limits for the email client you use and stick within those parameters.

Ditch the spam words

Hundreds of spammy words can get your message stuck in a spam filter. Here are some of the worst offenders to be on the lookout for:

  • Risk-free investment
  • No credit check
  • You’ve been selected
  • Exclusive offer for you
  • Cash prize
  • You’re a winner!
  • Lowest insurance rates
  • Pre-approved
  • Lowest mortgage rates
  • Free gift
  • Don’t delete
  • Order now
  • Limited supply
  • Money-back guarantee
  • Cashback
  • Miracle cure
  • Boost your…
  • Increase your…
  • No strings attached
  • As seen on TV
  • Lowest price
  • Special promotion
  • Discount
  • Risk-free
  • Congratulations, you’re a winner
  • Lottery
  • Prize winner
  • Credit card offers
  • Lose weight fast
  • Make money fast
  • Amazing opportunity
  • Great offer
  • Incredible deal
  • Exclusive deal
  • Once in a lifetime
  • No obligation
  • Double your…
  • Cash bonus
  • Click here
  • Buy now
  • Guaranteed
  • Urgent
  • Act now
  • Free

Prune your email list

ISPs don’t want you emailing spam traps, and more so than that, you should have an active, engaged email following to maintain a good sender reputation. That’s why pruning your email list every few months is advisable.

Cutting out the dead weight, such as inactive and unresponsive email addresses, helps you maintain your sender reputation.

Reconsider your send frequency

If you’re being reported as spam for emailing too often, it might be worth checking in with your audience about what kind of send frequency they would prefer. You should also make it easy to opt out of your emails if people don’t want to see them.


When the mailer daemon strikes, it’s worth paying attention to. Unless you misspelled the email address you’re trying to reach, your message has bounced. A soft bounce usually resolves itself given time, but hard bounces are indicative of more serious issues.

Plus, receiving mailer daemon notices often enough is not a coincidence. You could be following spammy practices that prevent your emails from going through.

Small business owners must proactively manage their email communications to minimize bouncebacks and maintain a strong sender reputation.


What does a mailer daemon message mean?

Receiving a mailer daemon message means that your email cannot be sent at this time. The message will include an attachment detailing why your email failed.

Are mailer daemon messages a sign of a hacked email account?

No, not at all. They’re a warning sign of temporary errors and sometimes more serious, permanent internal problems.

How can I stop getting mailer daemon messages?

Triple-check your spelling when emailing a new address for the first time, limit your file size, use links instead of attachments, send your emails from a familiar address, and don’t send too many messages at once from that address.

What’s the difference between a hard bounce and a soft bounce?

Soft bounces are attributable to temporary errors that usually correct themselves with time, such as email server congestion. By comparison, a hard bounce is due to a permanent error and is often unfixable.

Can I recover an email address that consistently returns mailer daemon messages?

Possibly, but it depends on who’s on the other side of it.

How do mailer daemon errors affect my business’s email sender score?

Mailer daemon error codes can denote spammy practices, hurting your email sender score. However, receiving a daemon error in and of itself is not going to tank your sender reputation.

Is there a way to automatically manage or filter out mailer daemon messages?

No, as you need to see them to determine how to troubleshoot the issue.

How often should I clean my email list to prevent mailer daemon errors?

Get into the habit of cleaning up your email list twice per year and never going longer than a year.

What are the best practices for avoiding email attachments that cause bounce-backs?

First, try to avoid attachments altogether, if possible, as links are more trustworthy. If you must send an attachment, ensure it meets the size requirements. Name the file clearly so the recipient knows what it is before opening it, check it for viruses before sending it, and mention in your email that you’ve included an attachment.

How can I distinguish if a mailer daemon message is due to my email being marked as spam?

Review the code. If you get an error about prohibited sending, your messages may be marked as spam. However, you’ll have to do more investigating than that, such as using a tool that denotes spam scores and determining what yours is for your main business email address.

You can also check open rates. If they’re well under the 15% average, it’s probably because many people don’t get the chance to open them since your emails go to the spam filter.

About The Author

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top