Reference

SMTP Response Codes

Every status code a mail server can return, decoded. Find what went wrong, why, and how to fix it.

2xx Success Responses

The command completed successfully.

250

SMTP code 250 indicates a successful action

4xx Temporary Failures

The server encountered a temporary condition. Retry may succeed.

421

SMTP code 421 indicates a temporary issue with the server that prevents it from processing requests

450

SMTP code 450 is a generic temporary error and is used in a wide variety of conditions by the receiving server

451

SMTP code 451 indicates a temporary local error on the receiving server

452

SMTP code 452 is used to indicate that the recipient's email server is unable to accept the message due to a lack of storage space, too many recipients, or insufficient system resources

454

SMTP code 454 indicates an authentication issue, typically suggesting that the server is denying relay access due to a lack of proper authentication from the sender

455

SMTP code 455 is used to indicate a temporary issue with the server, where it suggests retrying the request at a later time

5xx Permanent Failures

The command failed. The error is not recoverable without changes.

530

SMTP code 530 typically indicates that authentication is required to proceed with the requested email operation

550

SMTP code 550 is a permanent failure notice, typically indicating that the recipient's email address is invalid or unavailable

552

SMTP code 552 generally indicates a failure due to storage issues or a limit imposed by the recipient's mail server

553

SMTP code 553 is used to indicate a rejection due to the recipient's email address being invalid, typically in scenarios where the address is not recognized by the recipient's mail server

554

SMTP code 554 is a permanent failure notice, indicating that the message has been rejected and will not be delivered

Understanding SMTP Codes

SMTP response codes are three-digit numbers returned during email transmission. The first digit indicates the class of response — success, temporary failure, or permanent failure. The second digit categorises the subject, and the third provides specific detail.

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