SMTP (Simple Mail Transfer Protocol) response codes indicate the status of email transactions. These codes help in diagnosing and fixing email delivery issues.
| Code | Description |
|---|---|
| 211 | System status or system help reply |
| 214 | Help message |
| 220 | Service ready |
| 221 | Service closing transmission channel |
| 235 | Authentication successful |
| 250 | Requested mail action okay, completed |
| 251 | User not local; will forward to forward-path |
| 252 | Cannot verify user, but will attempt delivery |
| Code | Description |
|---|---|
| 334 | Authentication challenge |
| 354 | Start mail input; end with <CRLF>.<CRLF> |
| Code | Description |
|---|---|
| 421 | Service not available, closing connection |
| 432 | Password transition is needed |
| 450 | Mailbox unavailable |
| 451 | Local error in processing |
| 452 | Insufficient system storage |
| 453 | No mail found |
| 454 | TLS not available |
| 458 | Unable to queue messages |
| 459 | Node not allowed |
| Code | Description |
|---|---|
| 500 | Syntax error, command unrecognized |
| 501 | Syntax error in parameters |
| 502 | Command not implemented |
| 503 | Bad sequence of commands |
| 504 | Command parameter not implemented |
| 521 | Machine does not accept mail |
| 530 | Must issue a STARTTLS command first |
| 534 | Authentication mechanism too weak |
| 538 | Encryption required |
| 550 | Mailbox unavailable |
| 551 | User not local; try forward-path |
| 552 | Exceeded storage allocation |
| 553 | Mailbox name not allowed |
| 554 | Transaction failed |
ESMTP codes (like 5.1.1) provide more detail. Format: X.Y.Z
Official reference: IANA SMTP Enhanced Status Codes
