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.
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.