Email Bounce Code Decoder
Explain 4xx/5xx SMTP bounce codes and how to fix them.
When mail bounces, the sender gets a failure notice containing an SMTP code like 550 5.1.1, 421 or 5.7.1. Paste the full bounce message and this tool finds the codes and explains whether the issue is temporary or permanent, plus the cause and how to fix it.
It helps you tell whether undelivered mail is a recipient-address problem or a sender reputation/auth problem.
5.1.1PermanentBad destination mailboxRecipient address doesn't exist. Check for typos or closed accounts.
550PermanentMailbox unavailable / rejected (permanent)Address doesn't exist or rejected by policy. Often a typo or closed account.
4xx (temporary) vs 5xx (permanent)
4xx is retried automatically and often delivers later (e.g. greylisting). 5xxwill keep failing until you fix the address or policy issue.
Frequent 5.7.x rejections?
Check your sending domain's SPF/DKIM/DMARC, sending IP reputation and reverse DNS (PTR). Most are sender-trust issues.
Frequently asked questions
Is 550 different from 550 5.1.1?
550 is a generic permanent rejection; 5.1.1 specifically means 'no such recipient.' When an enhanced code (x.x.x) is present, prefer it.
Will resending work?
For 4xx, automatic retries usually deliver. For 5xx, resending gives the same result until you fix the cause.
My mail keeps going to spam or getting rejected.
Check the sending domain's SPF/DKIM/DMARC and IP reputation. Failing authentication leads to 5.7.x rejections or spam folder placement.
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