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SMTP Status Codes
Look up SMTP reply codes and enhanced status codes, and parse SMTP responses for inconsistencies.
This tool handles input and primary processing in your browser and is designed not to send the input to QuickKit’s server.
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SMTP response (paste line)
SMTP reply code reference
| Code | Class | Meaning | RFC |
|---|---|---|---|
| 211 | 2xx Success | System status reply | RFC 5321 |
| 214 | 2xx Success | Help message | RFC 5321 |
| 220 | 2xx Success | Service ready | RFC 5321 |
| 221 | 2xx Success | Service closing transmission channel | RFC 5321 |
| 235 | 2xx Success | Authentication successful | RFC 4954 |
| 250 | 2xx Success | Requested mail action okay, completed | RFC 5321 |
| 251 | 2xx Success | User not local; will forward to forward-path | RFC 5321 |
| 252 | 2xx Success | Cannot verify the user, but will try to deliver | RFC 5321 |
| 334 | 3xx Continue | Server challenge response (AUTH) | RFC 4954 |
| 354 | 3xx Continue | Start mail input; end with CRLF.CRLF | RFC 5321 |
| 421 | 4xx Retry | Service not available, closing channel | RFC 5321 |
| 432 | 4xx Retry | A password transition is needed | RFC 4954 |
| 450 | 4xx Retry | Requested mail action not taken: mailbox unavailable | RFC 5321 |
| 451 | 4xx Retry | Requested action aborted: local error in processing | RFC 5321 |
| 452 | 4xx Retry | Requested action not taken: insufficient system storage | RFC 5321 |
| 454 | 4xx Retry | Temporary authentication failure | RFC 4954 |
| 455 | 4xx Retry | Server unable to accommodate parameters | RFC 5321 |
| 500 | 5xx Permanent | Syntax error, command unrecognized | RFC 5321 |
| 501 | 5xx Permanent | Syntax error in parameters or arguments | RFC 5321 |
| 502 | 5xx Permanent | Command not implemented | RFC 5321 |
| 503 | 5xx Permanent | Bad sequence of commands | RFC 5321 |
| 504 | 5xx Permanent | Command parameter not implemented | RFC 5321 |
| 521 | 5xx Permanent | Machine does not accept mail | RFC 7504 |
| 530 | 5xx Permanent | Authentication required | RFC 4954 |
| 534 | 5xx Permanent | Authentication mechanism is too weak | RFC 4954 |
| 535 | 5xx Permanent | Authentication credentials invalid | RFC 4954 |
| 538 | 5xx Permanent | Encryption required for requested auth mechanism | RFC 4954 |
| 550 | 5xx Permanent | Requested action not taken: mailbox unavailable | RFC 5321 |
| 551 | 5xx Permanent | User not local; please try forward-path | RFC 5321 |
| 552 | 5xx Permanent | Requested mail action aborted: exceeded storage allocation | RFC 5321 |
| 553 | 5xx Permanent | Requested action not taken: mailbox name not allowed | RFC 5321 |
| 554 | 5xx Permanent | Transaction failed | RFC 5321 |
| 555 | 5xx Permanent | MAIL FROM/RCPT TO parameters not recognized | RFC 5321 |
| 556 | 5xx Permanent | Domain does not accept mail | RFC 7504 |
How it works
SMTP reply codes identify server responses to commands. Enhanced status codes (RFC 3463) decode additional information. This tool includes basic reply codes from RFC 5321, RFC 4954 (AUTH), and RFC 7504, plus enhanced status codes from RFC 3463, RFC 4954, and RFC 7372. A parser extracts and validates codes from pasted SMTP responses and reports whether the basic and enhanced codes disagree on permanence. The free text sent by the server is shown verbatim; it is not standardized and this tool does not interpret it.