What this code means
The receiving SMTP server did not accept the message because the specified priority level was below the lowest level the server currently accepts. The leading digit 4 denotes a temporary failure, so a controlled retry is appropriate.
Technical meaning
The X.7.15 pattern means that the specified priority level is below the lowest priority acceptable to the receiving SMTP server. The standard identifies as one possible temporary condition a mode in which the server accepts only higher-priority messages for transfer and delivery while rejecting lower-priority messages. In the concrete 4.7.15 code, the leading digit assigns the result to the temporary class.
Delivery status
Code 4.7.15 describes a temporary failure of the current attempt; the condition may clear when the server's accepted threshold or operating mode changes. The code alone does not explain why the threshold applies, how long it will last, or whether the level specified by the sender is incorrect.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Operational recommendation: check suppression before every attempt, retain the full response and priority level used, then retry with backoff, jitter, idempotency, and a time or attempt limit. Do not raise priority merely to bypass the threshold without confirming the message's applicable policy. Stop after success, a permanent result, a suppression-state change that makes the recipient ineligible, or exhaustion of the limit.
Suppression decision
Operational recommendation: do not add the recipient address to a suppression list based on temporary code 4.7.15 alone, because the described condition concerns the priority level accepted by the server. Inspect the full response, priority used, bounded-retry outcomes, and other delivery events; suppress only after an independent permanent signal or when the applicable policy requires it.
Common causes
- The priority level specified for the message was below the lowest level accepted by the receiving SMTP server.
- The receiving server may have been operating temporarily in a mode that accepts only higher-priority messages while rejecting lower-priority messages.
Diagnostic steps
- Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 4.7.15 and that the basic reply is in the 4xx class; retain the complete response text.
- Correlate the response with the intended message, attempt time and stage, receiving endpoint, and priority level specified for that message.
- Use available sender, recipient, or provider configuration and logs to compare the level used with the accepted threshold and determine whether the server was in a temporary mode that restricted lower priorities; do not infer those details from the code alone.
- Check suppression before the next attempt, make a bounded retry without an unverified priority increase, and compare the outcome; stop after success, a permanent result, or exhaustion of the limit.
Actions by owner
Sender
- Confirm the message's intended priority and give the sender administrator the attempt time and complete response without repeatedly retrying it manually.
- Change the priority only when the confirmed message policy justifies another value, then report the outcome of one controlled attempt.
Sender administrator
- Retain the raw response, time, stage, receiving endpoint, and priority level used; check suppression and route the message through bounded retries with backoff and idempotency.
- Verify that the sending system specified the intended priority, and coordinate with the recipient administrator or provider to confirm the accepted threshold without raising the value on your own.
Recipient administrator
- If you manage the server that returned the code, inspect the configured threshold, operating mode, and priority-decision logs for the specified attempt.
- Clear the confirmed temporary condition within your control or provide safe context about the required threshold, then verify the result of a controlled attempt.
Provider
- If you operate a service involved in the attempt, inspect managed logs for the specified time, message, and endpoint, then confirm the level used, active threshold, and server-mode state.
- Clear the confirmed temporary condition in the managed layer or identify the permitted configuration while preserving the exact 4xx class, code 4.7.15, and safe diagnostic context in the response.
Sources and verification
The canonical T0 meaning of X.7.15 and its class assignment were verified against the IANA registry and RFC 2034, RFC 5248, and RFC 6710 as of July 17, 2026. Retry, suppression, diagnostic, and owner-action guidance is presented separately as operational advice; this record contains no provider-specific practice.
- iana-smtp-enhanced-status-codesT0 source
Enumerated Status Codes / X.7.15
- 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
- rfc6710T0 source
IANA registry reference for X.7.15
Last verified:
