What this code means
The receiving system permanently rejected the message because the SMTP client's IP address failed a reverse DNS check required by local policy. Do not retry the same attempt unchanged.
Technical meaning
The standard X.7.25 pattern means that an SMTP client's IP address failed a reverse DNS validation check, contrary to local policy requirements. Code 5.7.25 applies this meaning in the permanent-failure class.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. The code does not establish why reverse DNS validation failed, which layer needs correction, or the policy of any particular provider.
- 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 establishing the cause, making a confirmed correction, and checking the reverse DNS result; check suppression again first.
Suppression decision
Operational guidance: do not automatically suppress the recipient address or domain based on 5.7.25 alone. Inspect the complete response, event scope, DNS results, available logs, and delivery history, then base the suppression decision on the confirmed cause and applicable policy.
Common causes
- The SMTP client's IP address failed the reverse DNS check required by the local policy of the system that returned the code; the code alone does not identify the exact reason validation failed.
Diagnostic steps
- Inspect the raw SMTP response or nondelivery report and confirm the exact 5.7.25 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, SMTP client IP address, and system that returned the code.
- Use available logs, DNS results, and local-policy configuration to establish which reverse DNS check failed and why; do not assign a specific cause or provider practice from the code alone.
- Stop unchanged retries. After a confirmed correction, check the reverse DNS result and suppression again, then make at most one controlled attempt.
Actions by owner
Sender
- Do not manually retry the same unchanged message; give the sender administrator the complete response and attempt time.
Sender administrator
- Retain the complete response, correlate the attempt with the SMTP client IP address used, and inspect available logs, DNS results, and managed sending-layer configuration to establish the confirmed cause.
- Make only a confirmed correction, check the reverse DNS result and suppression again, and verify the outcome with one controlled attempt instead of retrying the unchanged message.
Recipient administrator
- If you manage the system that returned the code, inspect its reverse-DNS-validation logs and local policy for the specified attempt; correct a confirmed receiving-side problem or safely give the sender the context needed for remediation.
Provider
- If you operate a managed IP address, DNS, sending layer, or receiving-side validation, inspect the relevant logs and configuration, correct a confirmed problem in that layer or give the administrators precise diagnostic context, and do not trigger suppression from the code alone.
Sources and verification
The standard meaning of X.7.25 and its class-5 application were verified against the IANA registry and RFC 2034, RFC 5248, and RFC 7372 as of July 17, 2026. Diagnostic, remediation, retry, and suppression recommendations are separate operational guidance; this record contains no provider-specific practice.
- iana-smtp-enhanced-status-codesT0 source
Enumerated Status Codes / X.7.25
- 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
- rfc7372T0 source
IANA registry reference for X.7.25
Last verified:
