SMTP Delivery Protocol Status Codes
X.5 status codes describe SMTP protocol and transaction conditions such as command sequence, syntax, arguments, recipient limits, and AUTH exchange handling. The complete code determines whether the response is successful, temporary, or permanent.
A delivery-protocol subject describes the interaction between client and server, not a general inbox-placement or recipient-state result. Preserve the command context and exact reply before changing the SMTP client, authentication flow, or recipient batching.
Choose the next check
- Preserve command contextKeep the exact enhanced code, SMTP command sequence, server text, and authentication state from the failed transaction.
- Check sequence and syntaxDistinguish a command out of sequence from an invalid command argument or syntax condition.
- Check recipient and AUTH limitsToo many recipients and AUTH-response limits need client-side changes tied to the detailed response.
Common mistakes
- Treating a protocol response as evidence that a recipient mailbox is invalid.
- Changing authentication credentials when the exact reply identifies command sequence or syntax instead.
- Retrying an unchanged transaction without retaining command context.
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
Related resources
Guide
- Sending Reliability
Map SMTP 4xx/5xx protocol outcomes to retry vs permanent handling.
Incidents
- Bounces, Rejections, and Throttling
Operational triage when SMTP protocol replies defer or permanently fail sends.
- Queues, Webhooks, and Sending Reliability
Clear send-queue backlogs caused by repeated protocol deferrals safely.
- Transactional Email Outages
Keep time-sensitive transactional mail within a retry budget shorter than token lifetime.
Wojtek Blazalek
Email deliverability expert
Stuck on this error code? I help teams identify rejection causes and fix authentication and reputation, so email reaches the inbox.
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