Bounce message decoder
Paste the bounce-back message you got, or just the error code. The decoder explains it in plain English: what went wrong, whether it's permanent, and what to do next.
How bounce codes work
A bounce message usually contains two codes. The first, three digits like 550, is the basic SMTP reply. The second, like 5.1.1, is the more precise "enhanced status code".
- Codes starting with 2 mean success.
- Codes starting with 4 are temporary. Your server keeps trying for a while, often a day or more.
- Codes starting with 5 are permanent. The message was refused and won't be retried.
The most common bounce codes
Each code has its own page with causes and fixes in the SMTP error code library.
| Code | Meaning | Type |
|---|---|---|
550 5.1.1 | The mailbox doesn't exist | Hard bounce |
550 5.1.2 | The domain doesn't exist or can't receive email | Hard bounce |
552 5.2.2 | The mailbox is full | Soft bounce |
552 5.3.4 | The message is too large | Soft bounce |
550 5.7.1 | Refused by policy, often spam filtering or a blocklist | Block |
550 5.7.26 | Failed authentication (DMARC, SPF or DKIM) | Block |
421 4.7.0 | Temporarily slowed down: too much mail at once | Soft bounce |
450 4.7.1 | Temporarily refused, often greylisting | Soft bounce |
Providers word their messages differently, so the text after the code matters too. Read more in email bounce-back messages explained.
Hard bounces, soft bounces and blocks
A hard bounce means the address won't work, so remove it from your list. A soft bounce is temporary, so keep the address unless it keeps bouncing for weeks. A block is different: the problem is your sending setup or reputation, not the address, so fix the cause before you send again. See soft bounce vs hard bounce.
What the bounce message decoder does
When an email bounces, the message you get back is often full of codes and server jargon. This decoder reads the bounce for you and explains, in plain English, what went wrong, whether the problem is permanent, and what to do next. It also tells you whether to keep or remove the address from your mailing list.
Benefits
- Paste the whole message or just a codeYou can drop in the full bounce, a detailed code like 5.7.26, or a basic three-digit code like 452. The tool finds the code on its own.
- Works even without a clear codeIf the bounce has no code, the decoder looks for common wording such as "mailbox full", "user unknown", "relay denied" or a blocklist name like Spamhaus.
- Hard, soft or blocked at a glanceEach result is labeled as a permanent hard bounce, a temporary soft bounce, a block caused by your sending setup, or not a failure at all.
- Spots out-of-office repliesIf what you pasted is really an automatic "I'm away" reply, the tool says so, so you don't delete a good contact.
- Clear advice for your mailing listEvery result includes a line telling you whether to remove the address, keep it for now, or fix your sending setup first.
- Covers Gmail and Microsoft codesThe decoder knows many provider-specific codes, including Gmail rate limits and Microsoft 365 and Outlook.com blocks.
- Links to a full guideFor many codes, you get a link to a detailed page in the ListaFix SMTP error library with causes, examples and fixes.
- Shows every code it foundIf the bounce mentions more than one status code, the extra codes are listed so nothing gets missed.
Who it is for
- Small business owners who got an "Undeliverable" message and want to know if the address is wrong or their own email is the problem.
- Email marketers deciding which addresses to remove after a campaign.
- Office managers and IT helpers fixing a coworker's sending problem.
- Freelancers whose invoice or proposal bounced and who need to know whether to resend.
- Anyone who simply wants a bounce explained without reading technical server text.
How to use it
- Copy the bounce message from your inbox, or note the error code.
- Paste it into the box.
- Click Decode.
- Read what it means, what to do, and the advice for your mailing list.
Your privacy
The decoding runs entirely in your browser. The bounce text you paste is not sent to ListaFix or anyone else.
Questions people ask
- What does 550 5.1.1 mean?
- The email address doesn't exist on the receiving server. Check for a typo; if the address is right, the account was closed.
- What does 5.7.26 mean?
- Gmail and others rejected the message because it failed authentication: the sender's domain needs working SPF and DKIM that match the From address.
- Should I resend after a soft bounce?
- Your server already retries automatically. If it finally gives up, try again in a day or two.