TL;DR
Permanent for the unchanged attempt, but unspecified: diagnose before sending again.
What this code means
X.0.0 is the undefined enhanced-status pattern, used when no finer subject or detail is available. Class 5 makes this attempt permanent, not a diagnosis of an invalid recipient, policy rejection, or any other particular condition.
Technical meaning
Code 5.0.0 combines the X.0.0 undefined detail with permanent class 5. The basic reply text, SMTP stage, and logs carry the missing context.
Delivery status
The leading 5 means the unchanged attempt will not succeed. It does not establish why it failed or whether the recipient address is at fault.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Stop automatic and manual retries of the unchanged attempt. Submit again only after the full response identifies a condition that has been corrected.
Suppression decision
Do not automatically suppress an address or domain from 5.0.0 alone. Apply policy only after inspecting the response, event history, and established cause.
Common causes
- The responder supplied only a permanent class and no more specific enhanced status.
Diagnostic steps
- Retain the raw response, SMTP stage, attempt time, and message identifier; determine whether a more specific reason appears in logs or provider reporting.
Actions by owner
Sender administrator
- Stop unchanged retries and trace the exact response through sending logs or webhooks before choosing a repair or suppression action.
Provider
- If you operate the responding system, inspect the transaction and return or document the more specific cause where available.
Sources
These sources define what this enhanced status code means, mainly through the IANA registry and related RFCs. When provider examples appear on the page, they come from that provider's published documentation. Follow the links to read the original wording in context.
- SMTP Enhanced Status Codes — IANA registry of enhanced mail system status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- RFC 2034 — SMTP Service Extension for Returning Enhanced Error Codes — Defines how SMTP returns enhanced status codes to clients.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
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