What do the SMTP error / reply codes mean?
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Here is a reference table of SMTP error / reply codes and a brief description of their meaning:
| Code | Description |
|---|---|
| 211 | System status or system help reply. |
| 214 | Help message. |
| 220 | Domain service ready - ready to start TLS. |
| 221 | Domain service closing transmission channel. |
| 250 | OK - queuing for node started - requested mail action okay, completed. |
| 251 | OK - no messages waiting for node - user not local will forward to forward path. |
| 252 | OK - pending messages for node started - cannot VRFY user but will accept message and attempt delivery. |
| 253 | OK - messages pending - messages for node started. |
| 354 | Start mail input - end with |
| 355 | Octet-offset is the transaction offset. |
| 421 | Domain service not available - closing transmission channel. |
| 432 | A password transition is needed. |
| 450 | Requested mail action not taken: mailbox unavailable - ATRN request refused. |
| 451 | Requested action aborted - local error in processing - unable to process ATRN request. |
| 452 | Requested action not taken - insufficient system storage. |
| 453 | You have no mail. |
| 454 | TLS not available due to temporary reason - encryption is required for requested authentication mechanism. |
| 458 | Unable to queue messages for node |
| 459 | Node not allowed: reason. |
| 500 | Command not recognized 'command' - syntax error. |
| 501 | Syntax error - no parameters allowed. |
| 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 - encryption required for requested authentication mechanism. |
| 534 | Authentication mechanism is too weak. |
| 538 | Encryption required for requested authentication mechanism. |
| 550 | Requested action not taken - mailbox unavailable. |
| 551 | User not local - please try forward path. |
| 552 | Requested mail action aborted - exceeded storage allocation. |
| 553 | Requested action not taken - mailbox name not allowed. |
| 554 | Transaction failed. |
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