Bounce / NDR Analyzer
Paste the full bounce message (the non-delivery report your mail server returned) and SysGlass decodes the SMTP reply code and the RFC 3463 enhanced status code, recognises common Gmail / Microsoft 365 / Yahoo patterns, and tells you the real reason it bounced — authentication, sender reputation, content, rate limiting, greylisting, a bad recipient, TLS, or DNS — with the exact remediation. Most admins only see 'it bounced'; this tells you why.
Reviewed by the SysGlass team · Updated July 21, 2026
What it checks
- The 3-digit SMTP reply code and whether it's permanent (5xx) or temporary (4xx, auto-retried)
- The RFC 3463 enhanced status code (e.g. 5.7.26, 5.1.1, 4.7.0) and what it means
- Provider-specific patterns from Gmail, Microsoft/Outlook and Yahoo/AOL
- The cause category: authentication, reputation/blocklist, content/spam, rate limit, greylisting, bad recipient, mailbox full, relay, TLS, DNS, reverse DNS
- The exact remediation for that category, linking to the relevant SysGlass tool
How to read the result
- Permanent (5xx) = the receiver rejected it outright; fix the cause before resending.
- Temporary (4xx) = a transient deferral (greylisting, rate limiting, mailbox full); your server retries automatically.
- Authentication = SPF/DKIM/DMARC failed or didn't align — run the Mail Deliverability Test.
- Reputation = the sending IP/AS is blocklisted or distrusted — check the Blacklist and ASN tools.
- Bad recipient = the address doesn't exist; remove it to protect your reputation.
Frequently asked questions
What does 550 5.7.26 mean?
Gmail returns 5.7.26 when a message is unauthenticated — neither SPF nor DKIM passes and aligns with your From domain (failing DMARC). Set up and align SPF/DKIM, publish DMARC, then resend.
What's the difference between a 4xx and a 5xx bounce?
4xx is a temporary failure — the receiver is deferring (greylisting, throttling, mailbox full) and your server retries automatically. 5xx is permanent — the message was rejected and won't be retried until you fix the cause.
My mail bounces with a Spamhaus / blocklist message — what now?
That's a reputation block. Check the sending IP with the Blacklist tool, request delisting where possible, fix whatever caused the listing (a compromised account or open relay), reduce complaints, and warm up. If the whole AS is listed, move to a better-reputation IP.
Does pasting a bounce store my email?
No. The analyzer only reads the bounce text you paste to classify the code — it isn't stored, and it's not your message content, just the delivery error.