SMTP Other or Undefined Status Codes
X.0.0 codes report a result class without a more specific subject condition. Keep the full SMTP reply or DSN because the same subject can record success, a temporary failure, or a permanent failure.
Other or undefined is a catch-all subject, not an explanation for why delivery changed. The leading digit supplies the reported outcome class, while provider text and delivery context are necessary to decide what happened and what to do next.
Choose the next check
- Keep the complete responsePreserve the basic SMTP reply, enhanced code, provider text, time, and message context before choosing a delivery action.
- Separate success from failureDo not read the shared X.0.0 subject as a rejection: 2.0.0 and 5.0.0 have different reported result classes.
- Investigate provider detailA catch-all code needs accompanying text and correlated events to narrow the actual condition.
Common mistakes
- Treating 2.0.0 as a rejection because it shares the X.0.0 subject.
- Inventing a detailed cause from the catch-all subject alone.
- Applying an unlimited retry or suppression rule without the complete response.
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.
- 2.0.0Other 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.Delivery status2xx · SuccessRecommended actionCheck the full context
- 4.0.0Other 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.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 5.0.0Other 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.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
Related resources
Guide
- Sending Reliability
SMTP 2xx/4xx/5xx taxonomy for class-only X.0.0 codes.
- Deliverability
Use enhanced codes; class alone is a coarse signal.
Incidents
- How should we interpret the SMTP response code for this failed send?
Classify 4yz vs 5yz before retry or repair.
Wojtek Blazalek
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