How to read a bounce code
A bounce line such as 550 5.7.26 has two parts. The first three digits are the SMTP reply code. The second part is the enhanced status code from RFC 3463, written as class.subject.detail.
| Starts with | Meaning | What your server does |
|---|---|---|
4 (421, 451 / 4.x.x) | Temporary failure: the message was valid, but a temporary condition stopped delivery. | Retries later on its own. Fix the cause before the retries run out. |
5 (550, 554 / 5.x.x) | Permanent failure: resending the message as it is won't work. | Gives up and returns the message to you. Something has to change first. |
The middle number is the subject. x.7.x is RFC 3463's security or policy class, which is why almost every authentication and spam rejection on this page is a 5.7 or 4.7 code. The last number is the provider's own detail, so the same code can mean different things at Gmail and at Microsoft.
Temporary and permanent pairs
Gmail often uses the same detail number twice: 421 4.7.27 rate-limits a message that failed SPF, and 550 5.7.27 blocks it. Treat a 4.7 code from Gmail as an early warning. The pages below cover both versions where Google documents both.
Questions
Where do I find the code in a bounce message?
Look for the line from the receiving server in the non-delivery report ("Remote server returned…", "Diagnostic-Code" or "The response was"). It starts with three digits such as 550, followed by a code such as 5.7.515.
Should I resend the message after a 5xx bounce?
Not until you've fixed the cause. A 5xx code means the receiver will refuse the same message from the same setup again. A 4xx code is retried by your server automatically.
Why isn't my Yahoo code here?
Yahoo's Sender Hub describes its errors by category (authentication failures, TS deferrals, PH content blocks) but doesn't publish the exact text of codes like TSS04. We only list codes whose wording we can quote from the provider. Yahoo's authentication rules are on our Yahoo requirements page.
Can a DNS check find every cause?
No. SPF, DKIM and DMARC records are public, so a check finds missing or broken records. Alignment, spam rate, sending volume and reverse DNS depend on your messages and IP addresses; each page says how to check those.