SMTP Mail System Status Codes
X.3 status codes describe the operating state or capacity of a destination mail system. The complete enhanced code distinguishes a successful system action, a current obstacle, and a permanent failure.
Mail-system status can report a general system outcome, changed requested priority, exhausted storage, a system that is not accepting network messages, or a message-size limit. The shared subject does not prescribe a retry or identify a recipient problem.
Choose the next check
- Identify the reported system stateKeep the exact reply and determine whether it reports a success, a temporary capacity condition, or a permanent outcome.
- Check receiving capacityStorage and message-acceptance conditions concern the receiving system; use the class to guide timing and escalation.
- Check message sizeA size-limit response requires inspecting the message as sent before another attempt.
Common mistakes
- Treating a destination system condition as evidence that a recipient address is invalid.
- Retrying a size-limited message without changing the condition that caused the limit.
- Reading a shared X.3 subject without checking the leading result class.
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.
- 2.3.0Other 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.Delivery status2xx · SuccessRecommended actionCheck the full context
- 2.3.6Requested 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.Delivery status2xx · SuccessRecommended actionCheck the full context
- 4.3.0Other 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.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.3.1Temporarily 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.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.3.2System 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.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 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.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 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.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 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.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
Related resources
Guide
- Sending Reliability
Retry 4xx mail-system; do not retry unchanged 5xx.
- List Management
4.3.x system outages are soft, not list-hygiene events.
Incidents
- How should we interpret the SMTP response code for this failed send?
Interpret class before blaming the recipient address.
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