What this code means
Code 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.
Technical meaning
The X.0.0 pattern is the only undefined status code and is used when only the result class is known. In the 4.0.0 variant, that result class is temporary failure.
Delivery status
This is a temporary status, not a permanent rejection. The code alone neither identifies the cause nor establishes that the recipient address is invalid.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Retry with exponential backoff and jitter, preserving idempotency and enforcing an attempt limit. Stop retrying after success or a more specific permanent status; route repeated 4.0.0 results for investigation.
Suppression decision
Do not add the address to a suppression list based on code 4.0.0 alone. Inspect the full response, attempt history, and other events; suppress only when an independent permanent signal or the applicable policy justifies it.
Common causes
- The receiving server reported a temporary failure without providing a more specific enhanced code.
- The response-processing system retained only class 4 or replaced a more specific status with the general 4.0.0 code.
Diagnostic steps
- Inspect the raw SMTP transaction response and confirm that it contains the exact enhanced code 4.0.0.
- Confirm that the basic SMTP reply is in the 4xx class and agrees with the enhanced 4.x.x class; note the SMTP stage and accompanying text.
- Trace response mapping in logs and webhooks to determine whether a more specific status was lost or normalized to 4.0.0.
- Compare the result of the next controlled attempt and stop automated retries if it succeeds or returns a permanent rejection.
Actions by owner
Sender administrator
- Route the message to a bounded retry queue with idempotency, exponential backoff, and jitter; do not retry indefinitely.
- Retain the raw response and investigate repeated 4.0.0 results; do not create a suppression solely from this code.
Provider
- Retain the full raw SMTP response and a more specific status code when one is available.
- Review normalization rules and the source of the temporary failure when available detail is replaced by the general 4.0.0 code.
Sources and verification
The standard meaning and class assignment were verified against the IANA registry and RFC 2034, RFC 3463, and RFC 5248. The absence of an accepted Tier-1 example describes provider-data coverage only. Retry, suppression, diagnostics, and owner actions are operational guidance, not facts asserted by the standards or any specific provider.
- iana-smtp-enhanced-status-codesT0 source
Enumerated Status Codes / X.0.0
- rfc5248T0 source
Section 2.1: registry fields and non-exclusive Associated Basic Status Code
- rfc2034T0 source
Section 4: enhanced status class agrees with SMTP reply class
- rfc3463T0 source
IANA registry reference for X.0.0
Last verified:
