TL;DR
A generic temporary failure has no stated cause. Retry with backoff and an attempt limit; do not suppress from 4.0.0 alone.
What this code means
4.0.0 is the class-4 form of X.0.0, the undefined enhanced-status pattern. It reports a temporary result without identifying a mailbox, network, policy, or content cause.
Technical meaning
X.0.0 is used when only the result class is known; 4.0.0 therefore denotes temporary failure.
Delivery status
Class 4 means this attempt may succeed later. It does not prove an invalid recipient or a permanent block.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Use idempotent retries with exponential backoff, jitter, and a time or attempt limit. Stop on success, a permanent result, or exhaustion of that limit.
Suppression decision
Do not suppress an address from 4.0.0 alone. Decide only from the full response history, an independent permanent signal, and applicable policy.
Common causes
- The receiver returned a temporary failure without a more specific enhanced code, or processing normalized richer detail to 4.0.0.
Diagnostic steps
- Retain the raw response, SMTP stage, and attempt time; determine whether a more specific status was lost before making another bounded attempt.
Actions by owner
Sender administrator
- Place the message in a bounded idempotent retry queue and retain the raw response.
Provider
- Preserve any available detailed status instead of replacing it with generic 4.0.0.
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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