Security, authentication and policy
Authentication, authorization, reputation, security controls, and receiving policy.
This category covers 32 reviewed codes concerning general security statuses, delivery and mailing-list authorization, the AUTH exchange and encryption requirements, message priority, RRVS checks, DKIM, SPF, reverse DNS, and ARC, as well as conditions involving null MX and REQUIRETLS. The set contains one success code (2.7.0), six temporary class-4 codes, and 25 permanent class-5 codes, so the shared subject does not determine the outcome. The complete code is essential for distinguishing a general or policy-limited security status from a specific authentication, authorization, validation, or other receiving-policy failure.
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.
- 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
- 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
- 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
- 4.7.12Authentication 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
Wojtek Blazalek
Email deliverability expert
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