Email Bounced Back: Error Messages and How to Fix Them
Did an email you sent bounce back? How to read a Mail Delivery Failed message, what the 550, 552 and 554 error codes mean and how to fix the usual causes.
An email bounced back notice tells you that a message you sent didn't reach the recipient. It usually arrives with the subject Mail Delivery Failed, Undelivered Mail Returned to Sender or Delivery Status Notification (Failure), from MAILER-DAEMON or Mail Delivery Subsystem. Read the message before you delete it: a single line usually tells you what the problem is and how to fix it.
This guide covers how to read bounce messages, what the most common error codes mean and what to do about each one.
How to read a bounce message
Look for the response from the receiving server inside the message. It's a line like this:
550 5.1.1 <alex@example.com>: Recipient address rejected: User unknown
- The first number (
550): The SMTP reply code. - The second number (
5.1.1): The enhanced status code; it describes the type of problem in more detail. - The explanation: The receiving server describes the error in its own words. This is often the most informative part.
Permanent and temporary errors
- Codes starting with 5 are permanent (hard bounce). The email won't be delivered; sending it again without changing anything won't work.
- Codes starting with 4 are temporary (soft bounce). The sending server automatically retries the email for a while. The message you get is often just a warning; if the retry period runs out, a second message tells you the email couldn't be delivered.
Common error codes
| Code | Meaning | What to do |
|---|---|---|
550 5.1.1 | The recipient address doesn't exist | Check the address; a typo is a very common cause |
552 5.2.2 | The recipient's mailbox is full | Reach the recipient another way, then send again |
552 5.3.4 | The email exceeds the size limit | Make the attachments smaller or send them as a link |
550 5.7.1 / 554 5.7.1 | The receiving server rejected the email by policy | Read the explanation; it may be spam, a blacklist or an authorization problem |
550 5.7.26 | Gmail rejected an unauthenticated email | Fix your SPF, DKIM and DMARC records |
421 4.7.0 and other 4.x.x | Temporary rejection or rate limit | Wait; the server retries automatically |
Providers can use different codes and explanations for the same situation, so always read the explanation next to the code.
The recipient address doesn't exist (550 5.1.1)
This is the most common error. There's a typo in the recipient's address, the address has been deleted or the domain's email has moved to another server. Check the address letter by letter. If the address is correct, reach the recipient another way and ask whether the address is still in use.
Authentication and spam rejections (5.7.x)
Codes starting with 5.7 are security and policy rejections. The explanation may contain:
- SPF, DKIM, DMARC or unauthenticated: The receiving server couldn't verify that the email really came from your domain. Large providers such as Gmail and Microsoft are rejecting unauthenticated email more and more often. Set up your domain's DNS records as described in our SPF, DKIM and DMARC guide.
- Spamhaus, blocked, blacklisted or listed: The sending server's IP address is on a blacklist. If you send email from your own server, check your IP address and request delisting; if you use an email provider, forward the message to your provider.
- Spam or content: The email's content was judged to be spam. Lots of links, shortened links, image-only emails and subject lines in capital letters increase the risk.
- Relay access denied: Your email client is trying to send email without authenticating to the outgoing server. Turn on authentication in the outgoing server (SMTP) settings; the details are in our IMAP vs POP3 article.
If your emails aren't bouncing but are landing in the recipient's spam folder, take a look at our Why do emails go to spam? guide.
Mailbox full and size limits
- The recipient's mailbox is full (
552 5.2.2): The problem is on the recipient's side. Reach the recipient another way; send the email again once they've freed up space in their mailbox. - The email is too large (
552 5.3.4): Attachments grow by about a third when they're added to an email; a 20 MB file takes up around 27 MB in an email. Send large files as a file sharing link.
When your own mailbox is full, emails sent to you bounce back too. If you use IMAP, your email takes up space on the server; delete or archive old emails with large attachments.
Domain not found
Phrases such as Domain not found, Host or domain name not found or No MX record mean there's a typo in the recipient's domain or the domain has no MX record to receive email. Check the domain. If the domain is yours, make sure its MX record points to the right server; we cover DNS records in our What is DNS? article.
If emails you didn't send are bouncing back
If you're getting bounce messages for emails you never sent, spammers may be using your address as a fake sender address. This is called backscatter.
- Check your sent items folder. If the emails aren't there, your account most likely hasn't been compromised.
- Change your password anyway, and turn on two-factor authentication everywhere you use it.
- Publish SPF, DKIM and a DMARC policy for your domain. Receiving servers then recognize and reject forged emails more easily.
If there are emails you don't recognize in your sent items, your account is being used: change your password right away and let your email provider know.
Nodesty mail hosting
Nodesty mail hosting plans include email addresses on your own domain, webmail and IMAP/POP3 access; incoming email is filtered by SpamExperts. When you add the domain verification and DKIM details from your panel to your DNS, the emails you send can be verified by receiving servers, which prevents authentication-related bounces. We cover setting up an email address on your own domain in our How to create a business email guide.
Frequently Asked Questions
What does Mail Delivery Failed mean?
It's an automatic message telling you that an email you sent couldn't be delivered to the recipient. The message contains the error code and explanation from the receiving server; that line tells you why the email couldn't be delivered.
What's the difference between errors starting with 5 and errors starting with 4?
Codes starting with 5 are permanent errors; sending the same email again without changing anything won't work. Codes starting with 4 are temporary errors; the sending server automatically tries to resend the email for a while.
I'm getting bounce messages for emails I didn't send. Why?
Spammers may have used your address as a fake sender address, and the bounce messages come back to you. If those emails aren't in your sent items folder, your account most likely hasn't been compromised. Publishing SPF, DKIM and DMARC records makes it harder for your address to be used this way.
I'm getting a recipient mailbox full error. What can I do?
The problem is on the recipient's side; emails can't be delivered until the recipient frees up space in their mailbox. Reach the recipient another way to let them know, and send the email again after a while.
Why did my email with an attachment bounce back?
The email may have exceeded the size limit of the receiving server or your own server. Attachments grow by about a third when they're added to an email. Send large files as a file sharing link.
How do I know if my server is on a blacklist?
If the bounce message mentions a blacklist name such as Spamhaus or words like blocked, blacklisted or listed, the sending server's IP address may be listed. Check the IP address on blacklist lookup sites and report it to your email provider.
About the author
Nodesty Team · Infrastructure & Support
The Nodesty team runs our servers, network and DDoS protection day to day and answers customer support requests around the clock. These guides come from that work.
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