Free tool
Email bounced back? Decode the bounce message
An email bounces back when the receiving server refuses it.
About this tool
About the email bounce decoder
The bounce message says why in two places: a code such as 550 5.1.1 or 421 4.7.0, and the provider's own wording after it. A 5xx code is permanent and a 4xx code is temporary; the second number group says what failed (x.1 the address, x.2 the mailbox, x.7 authentication or policy).
Paste the bounce above and the decoder matches it against Google's, Microsoft's and Yahoo's published code lists: what happened, whether to remove the address, and what to fix. It runs in your browser tab; the bounce is never uploaded.
How it works
Why emails bounce back: six causes
Every bounce falls into one of six groups, and each group has a different fix. The code narrows it down; the wording after the code settles it.
- Bad address (550 5.1.1, Microsoft 5.1.10 and 5.4.1 "Recipient address rejected"): the mailbox does not exist; remove it from the list and check the rest with the email list cleaner
- Full or inactive mailbox (452 4.2.2, 552 5.2.2): retry later, and drop the address if it stays full
- Authentication (550 5.7.26, 5.7.27, 5.7.30, Outlook.com 5.7.515, Microsoft 5.7.509): SPF, DKIM or DMARC failed for your domain; see 550 5.7.26, DMARC fail and DKIM fail
- Reputation or blocklist (550 5.7.1, 421 4.7.0, Microsoft 5.7.606 to 5.7.649): the receiver distrusts your IP or domain; see 421 4.7.0 and 550 5.7.1
- Rate limit (421 4.7.28, Microsoft 4.7.500 to 4.7.699): you sent faster than the receiver accepts from you; see 421 4.7.28
- A rule at the recipient (Gmail 550 5.7.1 "policy that prohibits", Microsoft 5.7.133, 554 5.7.1 relay access denied): the recipient's admin blocks outside mail or the domain's mail moved; remove the address
Background
Hard bounce, soft bounce and block
A hard bounce is a permanent failure for the address: 5.1.x, 5.2.1, 5.4.1 and similar. Retrying sends the same email into the same wall, and repeated hard bounces tell Gmail and Yahoo the list is unvetted. Remove the address everywhere, in every campaign and tool.
A soft bounce is a 4xx reply: the receiving server deferred the email and your server retries for a while. Deferrals for one address (full mailbox, recipient rate limit) clear on their own; deferrals for many addresses at once (421 4.7.0, 4.7.28) are about your sending and need lower volume.
A block is a 5.7.x refusal aimed at the sender. The address is fine; your domain, IP, authentication or content is the problem. Removing the address fixes nothing; the decoder says which part of your setup to change.
Background
Where to find the bounce message
- Gmail and Google Workspace senders: the reply from "Mail Delivery Subsystem"; the code and Google's wording sit under "The response from the remote server was"
- Outlook and Microsoft 365 senders: the "Undeliverable" report; open "Diagnostic information for administrators" for the Remote server line
- Sending tools: the bounce report arrives in the sending mailbox; open it there or in the tool's reply inbox and paste the line with the code
- Raw DSN: the Diagnostic-Code line (smtp; 550-5.7.26 ...) carries the receiving server's answer; the decoder reads it first
Background
When many emails bounce at once
Pause the campaign first. Then decode three or four bounces from different recipients: if they are all bad addresses, the list is the problem and bounce rate too high covers cleaning it; if they are blocks or rate limits, the problem is the sending side, and the domain health check and SPF, DKIM and DMARC checker show what the receivers see.
Bounces from one provider only (Gmail but not Outlook) point to that provider's view of your domain or IP. For Gmail, Google Postmaster Tools shows spam rate and delivery errors for your domain; for IPs you own, Microsoft SNDS shows Outlook.com's verdicts.
Background
Compared with bounce code lists
Bounce code glossaries list each code with one meaning. Gmail alone uses 550 5.7.1 for more than a dozen different texts, from low IP reputation to a missing Message-ID header, and 421 4.7.0 for at least seven. The decoder reads the wording as well as the code, so 550 5.7.1 "likely unsolicited" and 550 5.7.1 "policy that prohibits" get different fixes, and each answer links the provider's own documentation.
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Questions
What does it mean when an email bounces back?
The receiving mail server refused the email and your server sent you a report with the reason. A 5xx code is permanent; a 4xx code means the server will retry. The wording after the code is the provider's explanation.
Why did my email bounce back from Gmail?
Most often the address does not exist (550 5.1.1), SPF or DKIM failed (550 5.7.26, 5.7.27, 5.7.30), the sending IP or domain has a very low reputation (421 4.7.0, 550 5.7.1), or you sent too fast (421 4.7.28). Paste the bounce above to see which.
Should I resend an email that bounced?
Not a hard bounce or a block: the same email will fail the same way. A 4xx soft bounce is retried by your server automatically. After fixing a block (for example SPF or DKIM), send new emails; don't re-send the bounced ones in bulk.
What is the difference between a hard bounce and a soft bounce?
A hard bounce is permanent and about the address (it does not exist or is disabled): remove it. A soft bounce is temporary (full mailbox, rate limit, server busy) and your server retries. Blocks are permanent refusals aimed at the sender, and the fix is in your setup.
Is my bounce message uploaded anywhere?
No. The decoder runs in your browser tab and makes no network request with the text. The rules are downloaded once when you click Decode.
Why do I get bounces for addresses my verifier said were valid?
Verifiers cannot confirm every address: catch-all domains accept any test address, and Google Workspace hides whether many mailboxes exist. Also, some bounces are blocks aimed at your sender reputation, which no verifier can see.
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