Short answer: Paste a bounce (non-delivery report) or just the SMTP error line, and the explainer finds the status codes, such as 550 5.7.26 or 421 4.7.28, says whether the failure is permanent or temporary, recognises common Gmail, Microsoft, Yahoo, Spamhaus, cPanel, Exim and Postfix messages, and tells you the likely cause and the fix. Nothing you paste leaves your browser.
Built and tested in Chromium on 6 Oct 2026; runs entirely in your browser.
Table of Contents
How to use the bounce explainer
- Open the bounce you received (often titled “Mail delivery failed” or “Undeliverable”) and copy the whole text, or at least the line with the three-digit code.
- Paste it into the box. Headers such as
Final-Recipient,Remote-MTAandDiagnostic-Codeare picked up when they are present. - Press Explain bounce. Type
exampleto see a sample Gmail authentication bounce. - Follow the fixes in order; each links to our checker or guide for that problem.
What the output means
| Part | Meaning |
|---|---|
| Result | 5xx and 5.x.x codes are permanent: the message was not delivered and will not be retried. 4xx and 4.x.x codes are temporary: the sending server keeps retrying until it gives up. |
| SMTP reply code | The three-digit code (550, 421, 554…). |
| Enhanced status code | The class.subject.detail code (RFC 3463), for example 5.1.1. Subject 1 is addressing, 2 mailbox, 3 mail system, 4 network and routing, 5 protocol, 6 content, 7 security or policy. |
| Receiving provider | Detected from the text (gsmtp, outlook.com, yahoo…). Providers use some codes in their own way, and the tool shows both meanings. |
| Findings | Each recognised problem, its cause and what to do. |
| Status codes found | Every code, matched against the IANA SMTP Enhanced Status Codes registry. |
Gmail is the main example of provider-specific codes: in the IANA registry 5.7.27 means “sender address has null MX”, while Gmail uses 4.7.27 and 5.7.27 for SPF that did not pass. When the bounce comes from Google the tool explains Gmail’s meaning first, taken from Google’s Gmail SMTP error reference.
Examples
| Bounce text | What the tool says |
|---|---|
550-5.7.26 This mail has been blocked because the sender is unauthenticated … gsmtp | Gmail blocked an unauthenticated message: fix SPF, DKIM and DMARC for the From domain. |
421-4.7.28 Gmail has detected an unusual rate of unsolicited mail originating from your IP address | Rate limited: slow down and raise volume gradually. See the IP warm-up schedule. |
550 5.7.515 Access denied, sending domain example.com does not meet the required authentication level. | Outlook.com requires domains sending 5,000 or more messages a day to pass SPF and DKIM, publish DMARC (at least p=none) and align with the From domain. |
550 5.7.1 … part of their network is on our block list (S3150) | Outlook.com block list: fix the cause, then use Microsoft’s Outlook.com delisting form. |
550 5.7.606 Access denied, banned sending IP | Microsoft 365 banned the IP: request removal at sender.office.com. |
blocked using zen.spamhaus.org | Spamhaus listing: look the IP up at check.spamhaus.org; removal is free. |
550 5.1.1 … User unknown | The address does not exist. |
Domain example.com has exceeded the max emails per hour | cPanel’s hourly limit: see Exim outbound mail limits in WHM. |
Common mistakes
- Retrying a 5xx bounce unchanged. A permanent failure will fail again. Fix the cause first.
- Ignoring 4xx deferrals. Temporary errors that repeat for days, such as Gmail 421 4.7.28 or Yahoo [TSS04], mean the receiver distrusts your volume or reputation.
- Judging by the code alone. 5.7.1 can mean relay denied, a blocklist, spam content or a recipient policy. Read the text after the code; the tool does.
- Delisting before cleaning up. Blocklists relist quickly if the spam continues. Find the source first with our cPanel outgoing spam guide.
- Assuming the receiving server is right about Spamhaus. If the IP is not listed at check.spamhaus.org, the receiving server may be querying Spamhaus through a public DNS resolver, which Spamhaus does not answer correctly.
Official documentation: IANA SMTP Enhanced Status Codes · RFC 3463 · Gmail SMTP errors and codes · Microsoft: fix 550 5.7.515
Related: Email Header Analyzer · IP Blacklist Check: Bulk IPs, CIDR Ranges and Domains (33 Lists) · Mail server IP blacklisted: delisting runbook · Outgoing Spam cPanel: Find the Source and Stop It · Exim Outbound Mail Limits WHM: Stop Spam Runs Fast
See also: cPanel Exim Queue Stuck: Flush, Thaw and Delete Frozen Mail · Spamhaus 550 5.7.1 Blocked: Fix SBL, XBL, CSS, PBL and DBL
Frequently asked questions
What is the difference between a 4xx and a 5xx bounce?
4xx is temporary: the sending server keeps retrying, usually for several days. 5xx is permanent: the message will not be delivered unless you fix the cause and send it again.
What does 550 5.7.26 mean?
At Gmail it means the message was blocked because it was unauthenticated: SPF and DKIM did not pass, or the domain’s DMARC policy said to reject it. In the IANA registry 5.7.26 is “multiple authentication checks failed”.
What does 421 4.7.28 from Gmail mean?
Gmail detected an unusual rate of mail, often unsolicited, from your IP, domain or netblock and is rate limiting it. Slow down and increase volume gradually.
What does 550 5.1.1 mean?
The recipient address does not exist on the receiving server. Check the spelling and remove the address from your lists.
Is it safe to paste a bounce here?
Yes. The explainer runs in your browser and nothing is sent to srvScripts. Remove anything you would not want on screen before sharing a screenshot.