A bounce ("Mail delivery failed: returning message to sender") is an automatic reply explaining why a message could not be delivered. The most useful part is the three-digit SMTP code and the short text after it. Codes starting with 5 are permanent failures; codes starting with 4 are temporary and the server keeps retrying.
Before you start
- Open the bounce message and find the lines after "SMTP error from remote mail server" or "The following address(es) failed".
- Note the recipient address and the code, for example
550 5.1.1. - You can see the same information in cPanel Track Delivery.
Permanent errors (5xx)
| Code / text | Meaning | Fix |
|---|---|---|
550 5.1.1 User unknown, No such user | The address does not exist | Check the spelling. If it is your own domain, create the mailbox or a forwarder. |
550 5.7.1 ... SPF, SPF check failed | The sending server is not allowed by the domain's SPF | Fix SPF in Email Deliverability. |
550 5.7.26 ... DMARC, unauthenticated email | Gmail/Outlook reject mail that fails DMARC | Enable DKIM and SPF; send only From your own domain. |
550 ... listed at / blocked using | Sending IP is on a blocklist | Contact support with the full bounce. |
552 5.2.2 Mailbox full, quota exceeded | The recipient's mailbox is out of space | Recipient must free space. On your domain see fix a full mailbox. |
552 Message size exceeds | The message is too large | Send a download link instead of a big attachment. |
554 5.7.1 rejected, spam, content | The receiving filter rejected the content | Remove suspicious links and attachments; see why emails go to spam. |
550 Sender verify failed | The From address does not exist or cannot receive mail | Use a real mailbox as the sender. |
535 Incorrect authentication data | Your email app's SMTP login failed | Check the username (full address) and password. |
Temporary errors (4xx)
| Code / text | Meaning |
|---|---|
421 Try again later, 451 Temporary local problem | The receiving server is busy or greylisting. Retries usually succeed. |
451 ... rate limited, too many connections | The receiver throttles mail from this sender. |
Domain example.com has exceeded the max emails per hour | Your domain hit the hosting hourly sending limit. Remaining mail is queued and sent later. |
retry timeout exceeded | A temporary problem lasted too long and the server gave up. |
Note: A "delayed" or "warning" message is not a bounce; the server is still trying. Wait before resending, or the recipient may get the message twice.
Bounces for mail you did not send (backscatter)
If you receive many bounces for messages you never wrote, spammers are faking your address as the sender. Publish SPF, DKIM and a DMARC policy of quarantine or reject so receivers can discard those fakes. If Track Delivery shows these messages actually left your account, a mailbox or script is compromised: change passwords and contact support.