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Why Was the Email Rejected?

Email Bounce Codes and Classification

SMTP servers report delivery outcomes with reply codes. Enhanced Status Codes add a three-part X.Y.Z diagnosis, so you can separate a temporary obstacle from a permanent failure and decide what to inspect next.

Classify the outcome before acting

The leading digit is the first decision point. It does not, by itself, prove why the recipient should be suppressed.

2.x.xSuccess
The server reports a successful outcome. If it appears in a bounce flow, check the event classification before resending.
4.x.xTemporary failure
Delivery did not complete, but a controlled retry can be appropriate after checking timing, limits, routing, and the full response.
5.x.xPermanent failure
Do not repeat the same attempt unchanged. Correct the address, message, authentication, or policy condition first.
ContextSuppression needs context
A code is evidence, not the whole decision. Review the raw response, message history, provider evidence, and recipient state before suppression.

Browse by subject

Start with the middle digit when you know which part of delivery failed. Choosing a subject below filters the complete inventory.

Open category: Other and undefined
Open category: Addressing
Open category: Recipient mailbox
Open category: Mail system
Open category: Network, DNS and routing
Open category: Delivery protocol
Open category: Message content and format
Open category: Security, authentication and policy

Complete code inventory

Search the reviewed PL/EN catalog by exact code, title, explanation, or category. Filters work together.

  1. 2.0.02xx · SuccessOther undefined success statusCode 2.0.0 is a general success status: only class 2 is known, with no more specific status. It is not a message rejection.CategoryOther and undefinedRecommended actionCheck the full context
  2. 2.1.52xx · SuccessDestination address is validThe specified recipient mailbox address was found to be valid. This is a success confirmation, not a message rejection.CategoryAddressingRecommended actionCheck the full context
  3. 2.3.02xx · SuccessOther or undefined mail system statusThe destination system exists and normally accepts mail, but its condition caused a general DSN to be generated. Because the code begins with 2, it describes success, not a message rejection.CategoryMail systemRecommended actionCheck the full context
  4. 2.3.62xx · SuccessRequested priority was changedThe message was accepted for relay or delivery, but the requested priority—including a possible implied default—was not retained. The system reported a new priority instead. This is a success status, not a rejection.CategoryMail systemRecommended actionCheck the full context
  5. 2.5.02xx · SuccessOther or undefined protocol statusCode 2.5.0 is a general status concerning the protocol needed to pass the message to the next system. It provides no more specific detail, but the leading digit 2 means success, not rejection.CategoryDelivery protocolRecommended actionCheck the full context
  6. 2.6.42xx · SuccessConversion with data loss performedThe message was delivered, but delivery required a conversion in which some data was lost. This is a successful-result warning, not a rejection.CategoryMessage content and formatRecommended actionCheck the full context
  7. 2.6.82xx · SuccessRequired UTF-8 reply not permitted by the SMTP clientA reply needed to show the mailbox name would have to contain a UTF-8 string, but the SMTP client does not permit that form of reply. Code 2.6.8 reports this as a success status, not a rejection.CategoryMessage content and formatRecommended actionCheck the full context
  8. 2.7.02xx · SuccessOther 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.CategorySecurity, authentication and policyRecommended actionCheck the full context
  9. 4.0.04xx · Temporary failureOther undefined temporary statusCode 4.0.0 is a general temporary failure: only class 4 is known, with no more specific status. The message may be retried in a controlled way.CategoryOther and undefinedRecommended actionControlled retry
  10. 4.1.14xx · Temporary failureTemporary destination mailbox address problemThe receiving system did not find the specified mailbox during this attempt. Because the code begins with 4, this is a temporary failure: delivery may be retried in a controlled way, but the code alone does not yet establish that the address is permanently invalid.CategoryAddressingRecommended actionControlled retry
  11. 4.1.84xx · Temporary failureTemporary sender system address problemThis attempt reported a problem with the system identified by the sender address. Because the code begins with 4, it is a temporary failure: delivery may be retried in a controlled way, but the sender address and system configuration should be checked first.CategoryAddressingRecommended actionControlled retry
  12. 4.2.44xx · Temporary failureTemporary mailing-list expansion problemThe receiving system recognized the destination as a mailing list but could not expand it to recipients during this attempt. The code belongs to the temporary class, so delivery may be retried in a controlled way.CategoryRecipient mailboxRecommended actionControlled retry
  13. 4.3.04xx · Temporary failureOther or undefined temporary mail-system statusThe destination system exists and normally accepts mail, but a general problem in that system prevented delivery during this attempt. The leading digit 4 denotes a temporary failure, so delivery may be retried in a controlled way.CategoryMail systemRecommended actionControlled retry
  14. 4.3.14xx · Temporary failureTemporarily full mail systemThe mail system exceeded its available storage during this attempt. This is a system-wide resource problem, so an individual recipient may be unable to free space on their own. Because the code begins with 4, delivery may be retried in a controlled way.CategoryMail systemRecommended actionControlled retry
  15. 4.3.24xx · Temporary failureSystem temporarily not accepting network messagesThe host on which the recipient's mailbox resides is not currently accepting messages. Because the code begins with 4, this is a temporary failure and delivery may be retried in a controlled way.CategoryMail systemRecommended actionControlled retry
  16. 4.4.14xx · Temporary failureNo answer from remote hostAn outbound connection attempt received no answer from the remote system. The standard indicates that the system may have been busy or unable to accept a connection. The code belongs to the temporary class, so delivery may be retried in a controlled way.CategoryNetwork, DNS and routingRecommended actionControlled retry
  17. 4.4.24xx · Temporary failureBad connectionAn outbound connection was established, but the message transaction could not be completed over it. The standard points to a timeout or inadequate connection quality. The code belongs to the temporary class, so delivery may be retried in a controlled way.CategoryNetwork, DNS and routingRecommended actionControlled retry
  18. 4.4.34xx · Temporary failureDirectory 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. The code belongs to the temporary class, so delivery may be retried in a controlled way.CategoryNetwork, DNS and routingRecommended actionControlled retry
  19. 4.4.54xx · Temporary failureMail system congestionA mail system could not deliver the message because it was congested. The code does not identify which system or party is responsible for that condition. It belongs to the temporary class, so delivery may be retried in a controlled way.CategoryNetwork, DNS and routingRecommended actionControlled retry
  20. 4.5.04xx · Temporary failureOther or undefined temporary protocol statusA problem occurred with the protocol needed to pass the message to the next system, but no other detail code described it adequately. The leading digit 4 makes this a temporary result, so delivery may be retried in a controlled way.CategoryDelivery protocolRecommended actionControlled retry
  21. 4.5.14xx · Temporary failureTemporarily invalid protocol commandA mail system rejected a protocol command because it was issued out of sequence or was unsupported. The leading digit 4 classifies this concrete result as temporary, so delivery may be retried in a controlled way after the command context is checked.CategoryDelivery protocolRecommended actionControlled retry
  22. 4.5.34xx · Temporary failureToo many recipientsOne message specified more recipients than the protocol could handle in that attempt. The code belongs to the temporary class; normally, the remaining recipients should be separated and delivery to them retried in a controlled way.CategoryDelivery protocolRecommended actionControlled retry
  23. 4.5.44xx · Temporary failureTemporarily invalid command argumentsA mail system recognized the command but rejected its arguments as invalid. The leading digit 4 classifies this concrete result as temporary, so delivery may be retried in a controlled way after the arguments are checked and, when necessary, corrected.CategoryDelivery protocolRecommended actionControlled retry
  24. 4.7.04xx · Temporary failureOther 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.CategorySecurity, authentication and policyRecommended actionControlled retry
  25. 4.7.14xx · Temporary failureDelivery 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.CategorySecurity, authentication and policyRecommended actionControlled retry
  26. 4.7.124xx · Temporary failureAuthentication 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.CategorySecurity, authentication and policyRecommended actionControlled retry
  27. 4.7.154xx · Temporary failurePriority 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.CategorySecurity, authentication and policyRecommended actionControlled retry
  28. 4.7.164xx · Temporary failureMessage 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.CategorySecurity, authentication and policyRecommended actionControlled retry
  29. 4.7.244xx · Temporary failureSPF 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.CategorySecurity, authentication and policyRecommended actionControlled retry
  30. 5.0.05xx · Permanent failureOther 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
  31. 5.1.15xx · Permanent failureRecipient 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
  32. 5.1.35xx · Permanent failureInvalid 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
  33. 5.1.85xx · Permanent failureInvalid 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
  34. 5.1.105xx · Permanent failureRecipient 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
  35. 5.2.25xx · Permanent failureRecipient 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
  36. 5.2.35xx · Permanent failureMessage 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
  37. 5.3.05xx · Permanent failureOther 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
  38. 5.3.25xx · Permanent failureSystem 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
  39. 5.3.45xx · Permanent failureMessage 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
  40. 5.4.35xx · Permanent failureDirectory 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
  41. 5.5.05xx · Permanent failureOther 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
  42. 5.5.15xx · Permanent failureInvalid 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
  43. 5.5.25xx · Permanent failureProtocol 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
  44. 5.5.45xx · Permanent failureInvalid 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
  45. 5.5.65xx · Permanent failureAuthentication 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
  46. 5.6.35xx · Permanent failureRequired 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
  47. 5.6.65xx · Permanent failureMessage 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
  48. 5.6.75xx · Permanent failureNon-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
  49. 5.6.85xx · Permanent failureRequired 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
  50. 5.6.95xx · Permanent failureMessage 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
  51. 5.7.05xx · Permanent failureOther 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
  52. 5.7.15xx · Permanent failureDelivery 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
  53. 5.7.25xx · Permanent failureMailing 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
  54. 5.7.45xx · Permanent failureSecurity 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
  55. 5.7.85xx · Permanent failureAuthentication 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
  56. 5.7.95xx · Permanent failureAuthentication 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
  57. 5.7.105xx · Permanent failureEncryption 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
  58. 5.7.115xx · Permanent failureAuthentication 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
  59. 5.7.135xx · Permanent failureUser 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
  60. 5.7.145xx · Permanent failureTrust 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
  61. 5.7.155xx · Permanent failurePriority 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
  62. 5.7.165xx · Permanent failureMessage 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
  63. 5.7.175xx · Permanent failureMailbox 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
  64. 5.7.185xx · Permanent failureDomain 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
  65. 5.7.195xx · Permanent failureRRVS 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
  66. 5.7.205xx · Permanent failureNo 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
  67. 5.7.215xx · Permanent failureNo 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
  68. 5.7.225xx · Permanent failureNo 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
  69. 5.7.235xx · Permanent failureSPF 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
  70. 5.7.245xx · Permanent failureSPF 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
  71. 5.7.255xx · Permanent failureReverse 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
  72. 5.7.265xx · Permanent failureMultiple 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
  73. 5.7.275xx · Permanent failureSender 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
  74. 5.7.295xx · Permanent failureARC 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
  75. 5.7.305xx · Permanent failureREQUIRETLS 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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