What this code means
The 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.
Technical meaning
The X.3.1 pattern means that mail-system storage has been exceeded. The standard notes that the general semantics imply that an individual recipient may be unable to delete material to make room for additional messages. This detail is useful only as a persistent transient failure.
Delivery status
The leading digit 4 denotes a persistent transient failure: the current attempt failed, but the condition may clear. The code is not a permanent rejection and does not by itself identify the particular storage component or the owner of the system that returned it.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Operationally, retry with backoff, jitter, idempotency, and a time or attempt limit. Stop after success, a permanent result, or exhaustion of the limit; refer repeated 4.3.1 results to the recipient-system administrator or provider instead of retrying indefinitely.
Suppression decision
Operationally, do not add the address to a suppression list based on 4.3.1 alone. Inspect the full response and the history of bounded retries; suppress only after an independent permanent signal or when the applicable policy justifies it.
Common causes
- Mail-system storage was exceeded during the current attempt.
Diagnostic steps
- Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 4.3.1 and that the basic reply is in the 4xx class.
- Correlate the response with the intended message, attempt time, and system that returned it; do not infer the particular exhausted storage component from the code alone.
- Ask the recipient-system administrator or provider to inspect storage state for the same time and confirm whether capacity is available again.
- Compare the results of bounded retries and stop them after success, a permanent rejection, or the configured limit is reached.
Actions by owner
Sender administrator
- Retain the raw response and route the message through a bounded retry mechanism with backoff and idempotency.
- After the limit is exhausted, stop retrying and give the recipient-system administrator or provider the code, attempt time, and affected-flow identifier; do not suppress based on 4.3.1 alone.
Recipient administrator
- Inspect the mail system's storage state and availability for the failed-attempt time, then restore available capacity if the limit was exceeded.
- Verify the result after clearing the condition; do not direct the recipient to clean up their own mailbox without separate evidence that its resource is involved.
Provider
- If you manage the system that returned the error, inspect its storage use and limits for the specified time and clear the exceeded condition.
- After restoring the resource, confirm that a new attempt can be handled and preserve the exact status code in the response so the sender can stop retrying correctly.
Sources and verification
The canonical T0 meaning of X.3.1 and its class assignment were verified against the IANA registry and RFC 2034, RFC 3463, and RFC 5248 as of July 17, 2026. Retry, diagnostic, and suppression guidance is presented separately as operational advice.
- iana-smtp-enhanced-status-codesT0 source
Enumerated Status Codes / X.3.1
- 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.3.1
Last verified:
