What this code means
The message failed more than one authentication check required by local policy, so the system returned a permanent failure. Do not retry the same attempt unchanged.
Technical meaning
The standard X.7.26 pattern means that a message failed more than one authentication check, contrary to local policy requirements. The code does not specify which mechanisms failed. In code 5.7.26, the leading digit assigns the result to the permanent-failure class.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. The code confirms that multiple checks failed, but it does not identify their mechanisms, the precise cause, or the party responsible for the problem.
- 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 identifying the failed checks, making a confirmed correction, and checking suppression again.
Suppression decision
Operational guidance: do not automatically suppress the recipient address or domain based on 5.7.26 alone. Inspect the complete response, event scope, authentication results, and delivery history, then base the suppression decision on the confirmed cause and applicable policy.
Common causes
- More than one message authentication check failed, contrary to local policy requirements.
Diagnostic steps
- Inspect the raw SMTP response or nondelivery report and confirm the exact 5.7.26 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, identities used, and system that returned the code.
- Use available authentication results, logs, and configuration to establish which checks failed and which local-policy requirement was not met; do not derive those details from the code alone.
- Stop unchanged retries. After a confirmed correction, check authentication results and suppression again, then make at most one controlled attempt.
Actions by owner
Sender
- Do not manually retry the same unchanged message; confirm that the message and identities used are intended, then give the sender administrator the complete response and attempt time.
Sender administrator
- Retain the complete response, correlate it with the message and identities used, and inspect available authentication results, logs, and configuration to establish which checks failed.
- Make only a confirmed correction, check authentication results and suppression again, and verify the outcome with one controlled attempt.
Recipient administrator
- If you manage the system that returned the code, inspect its authentication-check logs and local policy for the specified attempt; correct a confirmed receiving-side problem or safely give the sender the details needed for remediation.
Provider
- If you operate a managed layer involved in the authentication checks, inspect its logs and configuration, correct a confirmed problem in that layer or give the appropriate administrators precise diagnostic context, and do not trigger suppression from the code alone.
Sources and verification
The standard meaning of X.7.26, its omission of the specific mechanisms, 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.7.26
- 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.7.26
Last verified:
