SMTP Error Code Lookup
Look up any SMTP reply code (550) or enhanced status code (5.1.1) and get its plain-English meaning and hard/soft bounce classification. Free reference tool.
Look up an SMTP error code
Enter a reply code (550), an enhanced status code (5.1.1), or a keyword from a bounce message.
What this SMTP code lookup does
Common use cases
- 1Decoding a bounce-back message
- 2Deciding whether to retry or drop an address
- 3Debugging SMTP rejections in mail logs
- 4Understanding a 550 or 5.7.1 rejection
- 5Training support staff on bounce handling
- 6Cleaning a list based on bounce reasons
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About SMTP error codes
When a mail server rejects or defers a message it replies with a 3-digit SMTP code and often an enhanced status code (the x.y.z form). The first digit tells you the class: 2xx success, 4xx temporary (retry), 5xx permanent (do not retry).
Reading these correctly is the difference between a healthy list and a damaged sender reputation. A hard bounce (e.g. 550 5.1.1, "user unknown") means the address is dead — remove it. A soft bounce (e.g. 4.2.2, "mailbox full") is temporary — retry later. This reference covers the codes you will actually encounter.
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What codes can I look up?
Enter a 3-digit SMTP reply code (like 550), an enhanced status code (like 5.7.1), or a keyword from a bounce message. The tool returns the meaning and whether it is a hard or soft bounce.
What is the difference between hard and soft bounces?
A hard bounce (5xx) is permanent — the address does not exist or is blocked, so remove it. A soft bounce (4xx) is temporary — mailbox full or server busy — so the sender should retry.
What does the first digit mean?
2xx means success, 4xx means a temporary failure you should retry, and 5xx means a permanent failure you should not retry.
What is an enhanced status code?
The x.y.z code (RFC 3463) that accompanies many replies, giving a more precise reason — for example 5.1.1 (bad mailbox) or 5.7.1 (delivery not authorised).
Why did I get a 550 or 5.7.1?
550/5.7.1 usually means the recipient does not exist or your mail was blocked by policy — failed SPF/DKIM/DMARC, a blacklist listing, or a relay restriction. The lookup explains each case.
Is this live data?
No — it is a reference of standardised codes (RFC 5321 / 3463). The meanings are fixed by the specifications, not measured live.