Delivery protocol
SMTP commands, transaction sequence, timing, and other delivery-protocol conditions.
This category covers ten reviewed codes concerning the delivery protocol flow. They address general protocol status, commands issued out of sequence or unsupported, command syntax errors, invalid arguments, too many recipients, and an overlong client response in the AUTH exchange for the selected SASL mechanism. The set contains one success code (2.5.0), four temporary codes (4.5.0, 4.5.1, 4.5.3, and 4.5.4), and five permanent codes (5.5.0, 5.5.1, 5.5.2, 5.5.4, and 5.5.6). The complete code therefore determines the result class: a shared protocol subject does not by itself mean rejection or establish whether a failure is temporary or permanent.
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.
- 2.5.0Other 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.Delivery status2xx · SuccessRecommended actionCheck the full context
- 4.5.0Other 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.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.5.1Temporarily 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.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.5.3Too 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.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.5.4Temporarily 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.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 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.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 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.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 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.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 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.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 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.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
Wojtek Blazalek
Email deliverability expert
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