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SMTP Security, Authentication and Policy Codes

X.7 status codes cover security, authentication, authorization, reputation, and receiving-policy conditions. The detailed code and provider text are essential: this category contains successful, temporary, and permanent results.

Security and policy status can concern authentication exchanges, authorization, encryption, message priority, DKIM, SPF, reverse DNS, ARC, null MX, or REQUIRETLS. Do not treat the subject as a generic authentication failure or as a reason to change recipient state.

Choose the next check

  1. Keep the complete policy responsePreserve the exact enhanced code, provider text, authentication results, sending identity, IP, and timestamp before changing delivery behavior.
  2. Check authentication and authorizationUse the detailed response to distinguish authentication, authorization, and encryption requirements from other policy conditions.
  3. Check provider policy and reputation contextRate, reputation, DNS, or provider policy results need the exact response and sending context before a controlled change.

Common mistakes

  • Calling every X.7 response an SPF or DKIM failure.
  • Suppressing recipients when the response identifies a sender-side authentication or policy condition.
  • Retrying unchanged traffic without correcting the detailed policy condition or checking provider guidance.
Use the exact X.7 response and authenticated sending context to repair the specific authorization, authentication, security, reputation, or policy condition.

Codes in this category

This inventory contains only reviewed codes assigned to this subject. Open a code to see its evidence, diagnostic steps and owner-specific actions.

Browse the complete catalog
  1. 2.7.0Other or undefined security status (success)The mail system reported a general security-related status that could not be described by a more specific code. The leading digit 2 denotes success, not message rejection.Delivery status2xx · SuccessRecommended actionCheck the full context
  2. 4.7.0Other or undefined temporary security statusThe message was returned because of a security-related problem, but the response does not describe it more precisely. The leading digit 4 denotes a temporary failure, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
  3. 4.7.1Delivery not authorized, message temporarily refusedThe system refused the current attempt because the sender was not authorized to send to the destination. The decision may result from per-host or per-recipient filtering. The leading digit 4 denotes a temporary failure, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
  4. 4.7.3Receiving domain's inbound queue temporarily over its rate quota (Outlook.com)The receiving system reports that the destination domain's incoming mail queue is currently receiving mail faster than its allotted rate quota and is not accepting messages at this time. The code belongs to the temporary class: the condition is tied to inbound volume, not to this message or this sender, so delivery may be retried once the queue has room again.Delivery status4xx · Temporary failureRecommended actionControlled retry
  5. 4.7.12Authentication mechanism transition requiredThe server replied to the AUTH command that the user must first transition to the selected authentication mechanism. The leading digit 4 denotes a temporary failure, so a controlled attempt may be made after the required transition is completed safely.Delivery status4xx · Temporary failureRecommended actionControlled retry
  6. 4.7.15Priority level is too lowThe receiving SMTP server did not accept the message because the specified priority level was below the lowest level the server currently accepts. The leading digit 4 denotes a temporary failure, so a controlled retry is appropriate.Delivery status4xx · Temporary failureRecommended actionControlled retry
  7. 4.7.16Message too big for the specified priorityThe server temporarily rejected the message because it is too big for the specified priority. A controlled retry may be made after the condition clears.Delivery status4xx · Temporary failureRecommended actionControlled retry
  8. 4.7.24SPF validation errorAn error occurred while SPF was evaluated for the arriving message, so the current attempt ended in a temporary failure. A controlled retry is appropriate, not unlimited retries.Delivery status4xx · Temporary failureRecommended actionControlled retry
  9. 4.7.26Mail rate-limited for missing SPF/DKIM authentication (Gmail)The receiving system reports that mail from this sender is being rate-limited because it arrived without a passing SPF or DKIM authentication result. The code belongs to the temporary class: once the sender's authentication is corrected, delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
  10. 4.7.28Sender IP temporarily rate-limited for unsolicited mail (Gmail)The receiving system reports that mail from this sending IP address is being rate-limited because Gmail judged an unusual portion of that traffic to be unsolicited. The code belongs to the temporary class: sending behavior and reputation may improve, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
  11. 5.7.0Other or undefined permanent security statusThe message was permanently rejected because of a security-related problem, but the response does not describe it more precisely. Do not retry the unchanged send.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  12. 5.7.1Delivery not authorized, message refusedThe system permanently refused the message because the sender was not authorized to send to the destination. Per-host or per-recipient filtering may produce this decision. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  13. 5.7.2Mailing list expansion prohibitedThe system permanently refused the message because the sender was not authorized to send to the intended mailing list. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  14. 5.7.4Security features not supportedThe message was permanently rejected because it used a security feature that could not be supported on the delivery protocol. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  15. 5.7.7Message rejected for duplicate authentication-results headersThe receiving system reports a permanent rejection under the enhanced code 5.7.7 because the message carried more than one copy of the same X-Original-Authentication-Results header. The registry pattern X.7.7 names a message-integrity failure; this catalog's only exact evidence for the concrete class-5 code comes from one relay that treats a duplicated authentication-results header as a sign of corruption or tampering and refuses the message outright.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  16. 5.7.8Authentication credentials invalidAuthentication failed because the credentials were invalid or insufficient. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  17. 5.7.9Authentication mechanism is too weakThe server permanently refused the AUTH attempt because the selected authentication mechanism was weaker than its policy permits for that user. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  18. 5.7.10Encryption neededThe server permanently refused the attempt to use the selected authentication mechanism because it requires an external strong privacy layer. Do not retry the unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  19. 5.7.11Authentication mechanism requires an encrypted connectionThe server permanently refused the AUTH attempt because the selected authentication mechanism may be used only over an encrypted SMTP connection. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  20. 5.7.13User account disabledAuthentication succeeded, but the account is disabled, so the server permanently refused the attempt. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  21. 5.7.14Trust relationship requiredThe submission server permanently refused the attempt because access to the message content requires a configured trust relationship with a third-party server. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  22. 5.7.15Priority level is too lowThe receiving SMTP server permanently refused the attempt because the message's specified priority was below the lowest level the server accepts. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  23. 5.7.16Message too big for the specified priorityThe server permanently rejected the message because it is too big for the specified priority. Do not retry the same attempt unchanged.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  24. 5.7.17Mailbox owner has changedThe receiving system permanently rejected the attempt because it determined that the mailbox had not remained continuously owned by the intended recipient since the time specified by RRVS. Do not retry the same attempt unchanged.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  25. 5.7.18Domain owner has changedThe receiving system permanently rejected the message and indicated that the owner of the recipient's domain had changed since the time supplied through RRVS. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  26. 5.7.19RRVS test cannot be completedThe receiving system permanently rejected the message because it could not complete the RRVS evaluation: the required timestamp had not been recorded. Do not retry the same attempt unchanged.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  27. 5.7.20No passing DKIM signature foundThe receiving system permanently rejected the message because it contained no DKIM signature that passed verification. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  28. 5.7.21No acceptable DKIM signature foundThe receiving system permanently rejected the message: at least one DKIM signature passed verification, but none was considered acceptable. Do not retry the attempt unchanged.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  29. 5.7.22No valid author-matched DKIM signature foundThe receiving system permanently rejected the message: at least one DKIM signature passed verification, but none had an identifier matching an author address in the From field. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  30. 5.7.23SPF validation produced a fail resultThe receiving system permanently rejected the message because its SPF check produced a fail result contrary to local policy. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  31. 5.7.24SPF validation errorSPF evaluation for the arriving message resulted in an error, and the system returned a permanent failure. Do not retry the same attempt unchanged.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  32. 5.7.25Reverse DNS validation failedThe receiving system permanently rejected the message because the SMTP client's IP address failed a reverse DNS check required by local policy. Do not retry the same attempt unchanged.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  33. 5.7.26Multiple authentication checks failedThe message failed more than one authentication check required by local policy, so the system returned a permanent failure. Do not retry the same attempt unchanged.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  34. 5.7.27Sender address has a null MXThe receiving system permanently rejected the message because the associated sender address has a null MX and the receiver rejects mail from such senders—for example, when it could not return a DSN. Do not retry the same attempt unchanged.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  35. 5.7.29ARC validation failureThis code may be returned when a message fails ARC validation. It denotes a permanent failure of the current attempt, so do not retry it unchanged.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
  36. 5.7.30REQUIRETLS support requiredThe message was received with a REQUIRETLS requirement but could not be forwarded because none of the destination SMTP servers provided that support. Code 5.7.30 denotes a permanent failure of the current attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged

Related resources

Guide

  • Deliverability

    SPF/DKIM/DMARC and related auth policy are required for inbox delivery.

Incidents

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