Understanding email error messages
When an email cannot be delivered you usually receive an automatic reply, often called a bounce or Mail Delivery Failure. It contains a code that tells you what went wrong.
How to read the code
- Codes starting with 4 (for example 451) are temporary. The sending server keeps trying for a while, so you often do not need to do anything.
- Codes starting with 5 (for example 550) are permanent. The message was rejected and will not be retried.
The most common messages
| Message | What it means | What to do |
|---|---|---|
| 550 5.1.1 User unknown / Recipient not found | The address does not exist, often a typing mistake. | Check the spelling of the address and resend. |
| 552 Mailbox full / over quota | The recipient’s mailbox has no space left. | Ask them to free up space. If it is your own mailbox, see What to do when your mailbox is full. |
| 550 Relay access denied / Authentication required | Your mail app is not logging in to the outgoing (SMTP) server. | Turn on “My outgoing server requires authentication” and use your full email address and password. See Email server settings. |
| 550 / 554 Message rejected as spam or blocked | The receiving server thinks the message looks like spam or your domain is not trusted. | Set up SPF, DKIM and DMARC: Stop your emails landing in spam. |
| Domain not found / 5.4.x | The part after the @ does not exist or cannot be found. | Check the domain spelling. |
| 552 Message too large | The attachment is bigger than the recipient allows. | Send a link to the file instead, or compress it. |
If you cannot work out the problem, send us the full bounce message (not a screenshot of just the heading) and we will look into it. See Troubleshooting: I cannot send or receive email.
Still need help?
Our team is happy to help. Please have your domain name or invoice number ready.
Was this article helpful?