TL;DR
Permanent refusal: the message priority is below what the receiving server accepts. Raise priority or confirm server conditions changed before retrying. A policy threshold issue with limited direct reputation impact unless sent at scale by mistake. Do not retry unchanged.
What this code means
Priority declared on a message can sit below the minimum the recipient SMTP system will honor for transfer; registry pattern X.7.15 then surfaces as code 5.7.15 in the security and policy subfamily. The first digit classifies the outcome as permanent in the enhanced status register, even though the standard notes the threshold may reflect a temporary operating mode that favors higher-priority traffic. The status names a priority-policy mismatch; it does not prove invalid addressing or an authentication failure. It also omits the accepted floor, the reason the rule is active, and any duration for a restricted mode.
Technical meaning
When the specified priority level is below the lowest priority acceptable to the receiving SMTP server, X.7.15 applies. The standard allows that the cause may be a temporary server mode in which only higher-priority messages are accepted for transfer and delivery while lower-priority messages are rejected, but code 5.7.15 reports the result in the permanent class.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. The code does not state the accepted threshold, why it applies, or how long the server mode will last, so those details must not be inferred from the code alone.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Operational guidance: stop automatic and manual retries of the same unchanged attempt. Consider a new, controlled attempt only after a confirmed correction to the intended priority or after the recipient or provider confirms that acceptance conditions changed; check suppression again first.
Suppression decision
Operational guidance: do not automatically suppress the recipient address based on 5.7.15 alone. Inspect the complete response, priority used, configuration, and event history, then make the suppression decision only from the confirmed cause and applicable policy.
Common causes
- The priority level specified for the message was below the lowest level accepted by the receiving SMTP server.
- The receiving server was operating 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 the exact 5.7.15 code and a basic reply in the 5xx 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 configuration and logs to check the intended priority, accepted threshold, and server operating mode; do not assume those values from the code alone.
- Stop unchanged retries; after a confirmed correction or change in conditions, check suppression again and compare the outcome of one controlled attempt.
Actions by owner
Sender
- Confirm the message's intended priority and give the administrator the attempt time and complete response without repeatedly retrying it manually.
Sender administrator
- Check the priority set by the sending system, retain the attempt context, and coordinate confirmation of the accepted threshold; make a new attempt only after a confirmed correction or change in conditions.
Recipient administrator
- If you manage the server that returned the code, inspect its priority threshold, operating mode, and logs for the attempt, then correct a confirmed problem or safely state the applicable requirement.
Provider
- If you operate a service involved in the attempt, inspect managed logs and configuration for that attempt, confirm the threshold and server mode, and correct only a problem in the managed layer.
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 6710 — SMTP Extension for Message Transfer Priorities — Defines message transfer priorities and related status codes.
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