SMTP Bounce Code Decoder
How to use this tool
Enter a code directly
Type a 3-digit SMTP code like 550 or an enhanced status code like 5.1.1 into the input and click Decode.
Paste the full NDR message
Got a full bounce-back email? Paste the entire message body. The tool will extract the error code automatically and identify it for you.
Understand hard vs soft bounces
Hard bounces (5xx) are permanent — remove those addresses. Soft bounces (4xx) are temporary and usually resolve on retry.
What this tool does
It identifies SMTP status codes from 4xx temporary errors to 5xx permanent failures, and maps common enhanced codes (5.1.1, 5.2.2, etc.) to a plain-English explanation. Each result includes the bounce type, a description of the cause, and the recommended next step for senders and developers.