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5.x.x — Permanent failure codes

Class 5 enhanced status codes mark a permanent failure. Do not repeat the same attempt unchanged; correct the address, message, authentication or policy condition first.

A leading 5 is a hard failure for the attempt as sent. Suppression may be justified, but a code is evidence, not the whole decision — review the raw response, provider evidence and recipient state before acting.

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.

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  1. 5.0.0Other undefined permanent statusCode 5.0.0 is a general permanent failure: only class 5 is known, with no more specific status. Do not retry the same unchanged attempt.CategoryOther and undefinedRecommended actionDo not retry unchanged
  2. 5.1.1Recipient mailbox does not existThe specified recipient mailbox does not exist. This is a permanent failure, so do not retry delivery to the same unchanged address.CategoryAddressingRecommended actionDo not retry unchanged
  3. 5.1.3Invalid recipient address syntaxThe message was permanently rejected because the recipient mailbox address has invalid syntax. Retrying with the same address will not remove the error.CategoryAddressingRecommended actionDo not retry unchanged
  4. 5.1.8Invalid sender system addressThe system identified by the sender address does not exist or cannot accept return mail. Code 5.1.8 is a permanent failure, so do not retry the unchanged send.CategoryAddressingRecommended actionDo not retry unchanged
  5. 5.1.10Recipient address points to a null-MX domainThe recipient address is associated with a domain marked by a null MX as not accepting mail. This is a permanent failure, so do not retry delivery to the same unchanged address.CategoryAddressingRecommended actionDo not retry unchanged
  6. 5.2.2Recipient mailbox is fullThe recipient mailbox has exceeded its assigned quota or reached physical capacity. Code 5.2.2 is a permanent failure, so do not retry the same unchanged send.CategoryRecipient mailboxRecommended actionDo not retry unchanged
  7. 5.2.3Message exceeds the mailbox limitThe message was permanently rejected because its length exceeds the administrative limit for the recipient's specific mailbox. Do not retry the unchanged send.CategoryRecipient mailboxRecommended actionDo not retry unchanged
  8. 5.3.0Other or undefined permanent mail-system statusThe destination system exists and normally accepts mail, but a general problem in that system caused a permanent failure. Do not retry the same unchanged attempt.CategoryMail systemRecommended actionDo not retry unchanged
  9. 5.3.2System not accepting network messages — permanent failureThe host on which the recipient's mailbox resides is not accepting messages and rejected the current request as a permanent failure. Do not retry the unchanged send.CategoryMail systemRecommended actionDo not retry unchanged
  10. 5.3.4Message exceeds the system size limitThe message was permanently rejected because it is larger than the system's per-message size limit. Do not retry the same unchanged send.CategoryMail systemRecommended actionDo not retry unchanged
  11. 5.4.3Directory server failureThe network system could not forward the message because a directory server was unavailable. Inability to connect to an Internet DNS server is one standards example, but the code alone does not prove that the problem involved DNS. Class 5 makes this a permanent result, so do not retry in the unchanged context.CategoryNetwork, DNS and routingRecommended actionDo not retry unchanged
  12. 5.5.0Other or undefined permanent protocol statusA permanent problem occurred with the protocol needed to pass the message to the next system, but no more specific code described it adequately. Do not retry the unchanged send.CategoryDelivery protocolRecommended actionDo not retry unchanged
  13. 5.5.1Invalid protocol commandA mail system permanently rejected a protocol command because it was issued out of sequence or was unsupported. Do not retry the same unchanged attempt.CategoryDelivery protocolRecommended actionDo not retry unchanged
  14. 5.5.2Protocol command syntax errorA mail system permanently rejected a mail transaction protocol command because it could not interpret it: the syntax was wrong or the command was unrecognized. Do not retry the same unchanged attempt.CategoryDelivery protocolRecommended actionDo not retry unchanged
  15. 5.5.4Invalid command argumentsA mail system permanently rejected a valid protocol command because its arguments were invalid. Do not retry the same attempt without changing the arguments or conditions that caused the rejection.CategoryDelivery protocolRecommended actionDo not retry unchanged
  16. 5.5.6Authentication exchange line is too longThe server permanently rejected an authentication attempt because the client's response was longer than the buffer available for the selected SASL mechanism. Do not retry the same unchanged attempt.CategoryDelivery protocolRecommended actionDo not retry unchanged
  17. 5.6.3Required conversion is not supportedThe message failed permanently because forwarding it required a content conversion that a host in the path could not practically perform. Do not retry the same unchanged attempt.CategoryMessage content and formatRecommended actionDo not retry unchanged
  18. 5.6.6Message content is not availableThe attempt failed permanently because the message content could not be fetched from a remote system. Do not retry the same unchanged attempt.CategoryMessage content and formatRecommended actionDo not retry unchanged
  19. 5.6.7Non-ASCII address is not permitted for the sender or recipientA system permanently rejected a MAIL or RCPT command because the sender or recipient address contained non-ASCII characters that were not permitted in that context. Do not retry the same unchanged attempt.CategoryMessage content and formatRecommended actionDo not retry unchanged
  20. 5.6.8Required UTF-8 reply not permitted by the SMTP clientThe attempt failed permanently because showing the mailbox name required a reply containing a UTF-8 string, but the SMTP client did not permit that form of reply. Do not retry the same unchanged attempt.CategoryMessage content and formatRecommended actionDo not retry unchanged
  21. 5.6.9Message with a UTF-8 header cannot be transferredThe message was permanently rejected after its data had been transmitted because it could not be transferred with its UTF-8 header to one or more recipients. Do not retry the same unchanged attempt.CategoryMessage content and formatRecommended actionDo not retry unchanged
  22. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  23. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  24. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  25. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  26. 5.7.8Authentication credentials invalidAuthentication failed because the credentials were invalid or insufficient. Do not retry the same unchanged attempt.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  27. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  28. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  29. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  30. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  31. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  32. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  33. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  34. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  35. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  36. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  37. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  38. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  39. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  40. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  41. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  42. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  43. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  44. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  45. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged
  46. 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.CategorySecurity, authentication and policyRecommended actionDo not retry unchanged

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