Mailer Daemon Errors, Fixes & Email Delivery Checklist

Mailer Daemon Errors, Fixes & Email Delivery Checklist

Emails are woven into our daily lives, but an unexpected "mailer daemon" response occasionally stops us in our tracks. This isn't a digital ghoul but a computer program ensuring emails reach their destination.

It's a good thing—provided you know how to avoid the errors that trigger it.

It detects issues and quickly notifies the sender through an error message. While these alerts might seem technical, they're vital roadmaps guiding us on email issues.

Remarkably, 19% of emails never reach their intended recipient. By understanding mailer daemon messages, we can better navigate email challenges, ensuring our communications are timely and effective.

2023 email deliverability rates

Mailer daemon errors and failures

What is a mailer daemon error?

When we send emails, we anticipate smooth delivery to the recipient. However, sometimes, the communication fails.

If the email cannot be delivered, the recipient's mail server, the technical infrastructure handling email traffic, promptly dispatches a mailer daemon message to the sender. This error message, though sometimes wrapped in technical jargon, is crucial. 

Ready to get busy?🕴️💰
Send professional transactional emails and attachments from inside your apps.

It will highlight one or more of a myriad of issues, from glitches on the mail server to invalid addresses that don't exist on the recipient's end. Given our increasing reliance on emails, understanding these messages becomes paramount.

Since you're reading this, you've probably already felt the disappointment of a crucial business proposal or a heartfelt message lost in the void due to unforeseen errors.

For key strategies to prevent this issue before it happens, watch the following video by Jun Wu.


Read our last post on email delivery subsystem, as well as out post on the importance of using email subdomains for your marketing emails.

Why call it mailer "daemon"?

Peek behind the curtain of computer science, and you'll find "daemons." These aren't mythical entities but processes that operate silently in the background.

Much like how artificial intelligence algorithms process vast amounts of data without our direct interference, daemons work autonomously. 

The mailer-daemon is a process dedicated to ensuring that messages find their way to the intended address. When the servers finish delivering your messages or encounter issues, this daemon steps up, ensuring you're informed.

Labeling it a "daemon" is apt not because it's somewhat bothersome, but because of its behind-the-scenes role in email communication.

talking to the mailer daemon

By understanding mailer daemons and their error messages, you'll be better equipped to handle challenges like spam blasts or password breaches that might compromise your mail password, ensuring your communications remain secure and effective.

Error 1: user unknown/address not found


You've just crafted an important email, clicked "send" and, almost instantly, you're met with a response—but it isn't from your anticipated recipient.

Instead, it's from the mailer daemon, informing you that the user is unknown or that the address cannot be found.


550 User Unknown

mailer daemon error 550 user unknown


When you encounter this error, it's known as a "hard bounce". In email terminology, a bounce is a returned message. This type of hard bounce is clear-cut: the address doesn't have a valid point of delivery on the email server. 

This is likely due to one or more of the following reasons—maybe there's a typo in the address, perhaps the email account was deleted, or possibly it never existed in the first place.


Before resending:

  • Take a moment to double-check.

  • Carefully review the recipient's email address for any discrepancies.

  • If you've got the address from the recipient directly, reach out through another communication channel to confirm its accuracy.

Sometimes, a simple oversight like a missed letter or misplaced dot can be the culprit. Always ensure you're sending to a valid and correct address to avoid further complications.

Error 2: mailbox full


In the lightening-paced world of the internet, emails fly back and forth constantly.

You've sent an important email filled with essential details. Yet, instead of the usual acknowledgment or reply, you're confronted with a response from the mailer.

Your email wasn't delivered—and the reason? The recipient's digital mailbox on their email server is full.


552 Mailbox Full

mailer daemon error code 552 mailbox full


This is what email experts refer to as a "soft bounce. Unlike a hard bounce where the address doesn't exist, the problem is more temporary here. Like all computers and services, the recipient's email server has a finite amount of storage. 

Once the email database hits its quota, it will not process incoming emails—at least, not until it's cleared out. If you imagine the mailbox as a physical desktop, it's so cluttered with papers (emails) that there's no space to place a new one.

desk with no more space on it


What's your best course of action? Contact the recipient through alternative means. Call them or send them a link via messaging services to notify them of the issue.

They must delete some emails, organize their links and files, or upgrade their storage capacity. 

Sometimes, they might not even know they've hit their limit, especially if they don't routinely scan their inbox. Ensuring they're informed helps both parties avoid guesswork and streamline communication.

Error 3: message size exceeds fixed maximum size


You've prepared an email, possibly with crucial documents or vivid images, and confidently hit "send". But instead of reaching its destination, it hits a digital wall. The culprit? The sheer weight of the content.


552 Message Too Large


Email servers, while impressive, have limits. Most email servers implement size restrictions on messages to safeguard against potential issues such as spam blast attacks or to manage storage efficiently. 

If an email, complete with its attachments, exceeds this size threshold, it won't get through. Sometimes, these size restrictions act as security measures, preventing potential threats like password breaches from large, suspicious files.


Don't worry; there are workarounds. Start by checking the size of your attachments. If they are the issue, consider compressing them. Compression tools can significantly reduce file sizes without compromising on quality. 

Alternatively, instead of attaching bulky files, upload them to a cloud service and provide the recipient with a link.

This ensures the email's delivery and provides an added layer of security by allowing recipients to change their email password if they feel the link might be compromised. 

Always ensure you inform your recipient about large files, especially if sharing via cloud links, so they know what to expect and can trust the source.

For dev to dev emails, and collaboration via email, one great way to send large files of any kind is to use a third-party cloud service like WeTransfer. Watch the following video by Dave's Tech Rescue to learn more.


Of course, this type of solution makes little sense for marketing and transactional email. For client communications, learn how to optimize images and media elements for email by exploring our relevant articles.

Error 4: service unavailable


You're ticking tasks off your to-do list, including sending out an essential email. Just when you expect a smooth sail, there's a halt. Your message can't be delivered because the recipient's mail server is unreachable.


421 Service Unavailable


Like any other infrastructure, servers need occasional downtime for various reasons. This error is indicative of a hiccup at the recipient's end. The mail server could be overloaded with traffic, leading to a short-term glitch. 

Alternatively, the server might be undergoing scheduled maintenance. It's akin to visiting a store only to find it temporarily closed for restocking or cleaning.

Sorry we're closed


Patience is the remedy here. Given the transient nature of this issue, your best move is to wait for a bit and then resend your email. Technology is fallible and, sometimes, servers need a brief respite to return to optimal function. 

However, if this error persists, contacting the recipient through another communication channel might be worthwhile, as would suggesting that they check in with their IT support.

They might not be aware of a prolonged issue, and your prompt might lead to a resolution for everyone trying to contact them.

Looking to integrate email into your apps?🤔📧
Mailer To Go is a streamlined cloud email client that allows seamless sending from inside your applications!

Error 5: relay access denied


In the case of this error, your email hasn't just been undelivered; it's been straight-out rejected.


554 Relay Access Denied


When faced with this error, there's often an issue with permissions. Relay, in email terminology, refers to the process of transferring emails from one server to another. For security reasons, not all servers are given this capability without checks. 

The rejection could be due to missing or incorrect authentication details. In other instances, the sender's server might not be on the approved list to relay messages through the recipient's mail server.

This mechanism is put in place to thwart potential misuse, like sending unsolicited bulk emails.


Begin by double-checking your settings. Have you entered the correct email password? Is your server configuration accurate? If you're using an email client, ensure you've set it up to authenticate when sending.

Sometimes, the solution is as simple as re-entering your password or updating server details.

Furthermore, authenticating your domain can prove helpful. It tells receiving servers that your domain is legitimate and has permission to send emails.

This can be done through SPF (Sender Policy Framework) or DKIM (DomainKeys Identified Mail), and a secure cloud email service like Mailer To Go.

mailer daemon frustrations

Mailer daemon spam

In the digital world, mailer daemons serve as vigilant guardians, ensuring your messages are rightly delivered or returned with the proper notifications.

However, just as every realm has its rogue elements, the world of emails is far from untouched by trickery. 

Spammers, ever crafty, have discovered ways to exploit the trusted name of the mailer daemon for their own nefarious objectives.

Spammers often cloak their spam blasts under the guise of legitimate-looking mailer daemon messages. Unsuspecting users may receive messages that appear to be bounce-backs but are malicious attempts to mislead or extract information.

address not found messages in inbox

To fortify yourself against such ploys, here's a checklist to adhere to:

If a mailer daemon message contains links, especially ones you weren't expecting or that seem out of context, do not click on them. Spammers often use these links to lead users to harmful sites or to download malware.

2. Cross-check with your sent folder

If you receive a bounce-back message for an email you don't recall sending, it might be a deceptive mailer daemon message. Verify with your sent messages.

If there's no trace of the email the daemon claims you sent, approach cautiously, and report the message!

3. Harness advanced technology

Use advanced artificial intelligence-powered spam filters. These filters learn and evolve, recognizing patterns and identifying even the most cleverly disguised spam.

4. Maintain robust password hygiene

Regularly updating your email password is an effective measure to stave off unauthorized access and potential password breaches.

If a spammer gains access to your account, they could send emails on your behalf, making mailer daemon spam seem more legitimate.

By being vigilant and incorporating these measures, you can effectively shield yourself from the deceptive tactics of spammers.

password hygiene

How do I know if MAILER-DAEMON is spam?

It's unfortunate but true that spammers sometimes disguise their unwanted emails under the guise of legitimate-sounding entities, like "MAILER-DAEMON." Here's how you can differentiate genuine bounce-back emails from spam:

1. Check the email you allegedly sent

The MAILER-DAEMON message should reference your email. Cross-reference with your sent folder. If there's no such email in your sent folder, the MAILER-DAEMON message might be a scam.

2. Inspect the details

Genuine bounce-back messages will typically provide specific reasons for the non-delivery, such as "User Unknown," "Mailbox Full," or "Domain Not Found." If the message is vague or doesn't give a clear reason, be suspicious.

A legitimate MAILER-DAEMON notice won't ask you to click a link to read your message or ask for personal details. If there's a link or an attachment, especially one prompting you to provide personal or financial information, steer clear.

reading an email with a suspicious link

4. Look for bad grammar or spelling

While a genuine bounce-back email occasionally contains errors, consistent poor grammar or spelling mistakes can be a red flag.

5. Examine the sender's email address

Look closely at the sender's email. Although spammers can spoof email addresses, they sometimes slip up. An email claiming to be a MAILER-DAEMON message from Gmail but sent from a random domain indicates spam.

6. Use email security features

Most modern email services have built-in security features that detect and quarantine phishing emails and spam. Ensure these are enabled—and use a secure email service like Mailer To Go.

7. Check the original email headers

Every email contains detailed headers that tell the email's journey from sender to recipient. If you're tech-savvy, you can inspect these headers to trace the origins of the email.

If something seems out of place or doesn't match where it claims to be from, it might be spam. If in doubt, always err on the side of caution.

If you believe you've received a spam email disguised as a MAILER-DAEMON message, delete it. It's better to be safe than sorry when navigating the often murky waters of the internet.

mailer daemon error delivery incomplete

Email delivery checklist

The art of sending emails may seem straightforward, but with the constant evolution of technology and protocols, ensuring successful email delivery has become more complex. 

Whether you're a business owner ensuring your newsletters reach your audience, or an individual trying to connect with friends, following an email delivery checklist can amplify your success rate.

1. Address accuracy

It may seem elementary, but it's pivotal. Before you send email, always double-check email addresses. A minor typo can send your email astray, causing unnecessary delays or miscommunications.

2. Mind the size 

Large attachments can be a barricade to successful delivery. Many email servers have size limits for attachments. Instead of sending colossal files directly, consider compressing them or using cloud storage services and sharing the link.

3. Server authentication

Authenticating your email server is akin to having an ID card in the digital space. Tools like SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) are vital.

They vouch for the legitimacy of your emails, decreasing the likelihood of them being flagged as spam.

To learn more about DKIM, watch the video below, by PowerDMARC.


4. Maintain a healthy inbox

Just as you'd clean out your physical mailbox to make room for new mail, regularly clearing out your inbox ensures you have space for incoming messages. It also helps in easily spotting and managing important emails.

5. Update server settings

Email server settings aren't always static. Service providers may update them for various reasons, including security enhancements. Make it a practice to periodically check and update your server settings per your email provider's recommendations.

Following this checklist optimizes your email delivery rates and fosters an efficient and secure emailing environment, ensuring your messages reach their intended destinations without hitches.

email marketer job well done

Wrapping up

As digital denizens, our quest isn't merely for convenience but also for the wisdom to navigate this realm with discernment and vigilance.

A password breach isn't merely an isolated glitch in the matrix; it's a glaring symptom of the vulnerabilities within our electronic sanctuaries. When such a breach unfolds, the floodgates to perils like spamming can swiftly open.

The best way to ensure you won't be getting Mailer Daemon responses is to follow the essential best practices outlined in this post, in conjunction with using a credible, secure, and trusted transactional and marketing email service like Mailer To Go.

About Mailer To Go! 📬
Send all kinds of email from inside your apps with Mailer To Go’s simpler, faster, better email service.

Frequently asked questions

What is a mailer daemon?

A mailer daemon is a program responsible for delivering your emails. It's also known as a "mail delivery agent." When you send email that can't be delivered for some reason, the mailer daemon sends a failure notification to the sender.

Why am I receiving emails from mailer-daemon?

If you're receiving emails from a mailer daemon, it means an email you tried to send couldn't be delivered. The mailer-daemon email should contain details about why the email couldn't be delivered.

What does a mailer-daemon failure notification look like?

A mailer-daemon failure notification typically starts with "MAILER-DAEMON" in the "From" field. The subject line usually contains something like "Mail delivery failed" or "Undelivered Mail Returned to Sender". The body of the email will contain details about why the email couldn't be delivered.

What is mailer daemon spam?

Mailer daemon spam occurs when spammers use your email address to send out spam emails. If these spam emails can't be delivered, you'll receive failure notifications from the mailer daemon, even though you didn't send the original emails. If you're receiving a lot of mailer daemon spam, it might be a sign that your email account has been compromised.

How can I prevent mailer daemon errors?

There are several steps you can take to prevent mailer daemon errors. These include verifying email addresses before sending, monitoring mailbox sizes, limiting email size, checking server availability, ensuring proper email relay configuration, avoiding spam-like behavior, keeping your email lists updated, using a reputable email service like Mailer To Go, and securing your email account.

What should I do if I receive a mailer daemon error?

If you receive a mailer daemon error, first read the error message to understand why the email couldn't be delivered. Then, take the appropriate action based on the error. This might involve correcting the recipient's email address, reducing the size of the email, or contacting your email service provider for assistance.

If you're receiving mailer daemon spam, you should secure your email account by changing your password and enabling two-factor authentication.


Boost Your Email ROI with Mailer To Go

Ready to boost email deliverability and clickthrough rates?

Start for free