What this code means
The receiving system did not find the specified mailbox during this attempt. Because the code begins with 4, this is a temporary failure: delivery may be retried in a controlled way, but the code alone does not yet establish that the address is permanently invalid.
Technical meaning
The X.1.1 pattern means that the mailbox specified in the address does not exist; for an Internet mail address, the part to the left of the “@” sign is invalid. The standards description notes that this detail is useful only for permanent failures, while the concrete 4.1.1 code combines it with temporary class 4 and therefore requires careful interpretation in the full response context.
Delivery status
The leading digit 4 denotes a persistent transient failure: the current attempt failed, but the condition may clear. It does not authorize unlimited retries; the unusual combination of a temporary class with the X.1.1 description should be confirmed in the raw response and attempt history.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Check the recipient address first and, if there is no confirmed error, schedule another attempt with backoff and an attempt limit. Stop automatic retries when the limit is reached or a permanent result arrives; do not assume that further attempts will repair an incorrect address.
Suppression decision
Do not add the address to a suppression list based on 4.1.1 alone. Decide only after evaluating the raw response, address accuracy, and the results of controlled retries; permanent confirmation of a nonexistent mailbox should be handled under the policy for permanent rejections.
Common causes
- The mailbox identifier to the left of the “@” sign is incorrect or does not correspond to a mailbox known to the receiving system.
- The receiving system did not find the mailbox during the current attempt but returned temporary class 4 instead of a permanent addressing code.
Diagnostic steps
- Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 4.1.1 and that the basic reply is in the 4xx class.
- Compare the recipient address with a trusted source value, paying particular attention to the mailbox identifier before the “@” sign; do not guess the correct address.
- Review the attempt history for the same message and address to determine whether the problem cleared, repeated, or was later reported as a permanent addressing failure.
Actions by owner
Sender
- Verify the address against a value supplied or confirmed by the recipient, and change it only when you have a reliable correction.
- Do not resend manually without a limit; if the address appears correct, leave the next attempt to the controlled retry mechanism.
Sender administrator
- Classify 4.1.1 as a temporary result and apply bounded retries with backoff while retaining the raw response and history of each attempt.
- After the attempt limit is exhausted or a permanent result appears, stop retrying and evaluate suppression from the full history rather than from code 4.1.1 alone.
Sources and verification
The X.1.1 meaning and class assignment come from the IANA registry and RFC 2034, RFC 3463, and RFC 5248, verified on July 17, 2026. The operational guidance on retries, suppression, diagnostics, and owner actions is separate advice, not standards facts or a description of provider practice.
- iana-smtp-enhanced-status-codesT0 source
Enumerated Status Codes / X.1.1
- 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.1.1
Last verified:
