Permanent or temporary
Codes starting with 5 are permanent: stop and fix the cause. Codes starting with 4 are temporary: the mail system will try again.
What does this bounce mean?
Paste the bounce message, or just the code, and read what it means and what to do about it.
What it means.
Two codes, one story
The three-digit code says what kind of problem. The dotted code after it says exactly which, such as 5.1.1 for an address that does not exist.
Prevent them
Most bounces are avoidable: check addresses when people sign up, set up SPF, DKIM and DMARC, and remove addresses that bounce.
Common questions.
What do the numbers in a bounce message mean?
The first, three-digit number (such as 550) is the SMTP reply code. The second, dotted one (such as 5.1.1) is the enhanced status code, which is more specific. The first digit tells you the most: 2 means success, 4 means a temporary problem you can retry, and 5 means a permanent one.
What is the difference between a 4xx and a 5xx error?
A 4xx error is temporary: the server could not take the message now, and mail systems retry for you. A 5xx error is permanent: retrying will not help, and doing it repeatedly can hurt your sending reputation.
Why did I get “550 5.1.1 user unknown”?
The address does not exist at that domain. Check it for typos. If it is right, the person has gone, so remove the address and do not send to it again.
Why did I get “550 5.7.1”?
The receiving server refused the message because of who sent it or how it looks: failing SPF, DKIM or DMARC, a blocklisted address, or a policy at the recipient. The words after the code usually say which. Check your domain with the free domain checker.
Should I keep retrying a bounced email?
Not for a permanent (5xx) bounce. Fix the cause or remove the address. For a temporary (4xx) one, let your mail system retry on its own, and slow down if the message says you are sending too fast.
Is what I paste sent anywhere?
No. Decoding happens in your browser.
Next: Check your domain · Check an email address · How to reduce your bounce rate · Analyze email headers
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