SMTP Reply codes are used to determine the error in the message, SMTP(Simple Mail Transfer Protocol) is a standard protocol for sending emails, the client sends and the server responds, a response is with the 3 digit code which contains the meaning for the response with the codes, find the below SMTP reply codes
SMTP Reply codes
| 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 node started. Requested mail action okay, completed. |
| 251 | OK, no messages waiting for node node. User not local, will forward to forwardpath. |
| 252 | OK, pending messages for node node started. Cannot VRFY user (e.g., info is not local), but will take message for this user and attempt delivery. |
| 253 | OK, messages pending messages for node 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 now |
| 452 | Requested action not taken: insufficient system storage. |
| 453 | You have no mail. |
| 454 | TLS not available due to temporary reason. Encryption required for requested authentication mechanism. |
| 458 | Unable to queue messages for node node. |
| 459 | Node 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 forwardpath. |
| 552 | Requested mail action aborted: exceeded storage allocation. |
| 553 | Requested action not taken: mailbox name not allowed. |
| 554 | Transaction failed. |
See Also
Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry – https://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml
IP subnet calculator – https://computercarriage.wordpress.com/2021/06/03/simple-ip-calculator-subnet-using-korn-shell-script-v1-0/
Shell script basics – https://computercarriage.wordpress.com/2020/12/06/shell-script-basics/
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