What this code means
The server permanently refused the AUTH attempt because the selected authentication mechanism was weaker than its policy permits for that user. Do not retry the same unchanged attempt.
Technical meaning
The standard X.7.9 pattern is a response to the AUTH command and means that the selected authentication mechanism is weaker than server policy permits for that user. The standard says the client should retry with a new mechanism; code 5.7.9 applies this detail in class 5.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. Repeating the attempt without changing the mechanism does not resolve the stated mismatch; the code alone does not identify an allowed mechanism, confirm incorrect credentials, or establish that the recipient address is invalid.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Operational guidance: stop automatic and manual retries with the same mechanism. Make a new, controlled attempt only after confirming and configuring another mechanism that complies with server policy; check suppression again before sending.
Suppression decision
Operational guidance: do not automatically add an address or domain to a suppression list based on 5.7.9 alone. Inspect the complete response, account or service scope, AUTH policy, and event history, then make the suppression decision according to the confirmed cause and applicable policy.
Common causes
- The client selected an authentication mechanism for the user that was weaker than server policy permits.
Diagnostic steps
- Inspect the raw SMTP response or attempt report and confirm that the enhanced code is exactly 5.7.9, that the basic reply is in the 5xx class, and that the code was returned to the AUTH command; retain the complete response text.
- Correlate the response with the attempt time, authenticating user, client, server endpoint, and selected mechanism; use available logs and policy to identify the refused mechanism and an allowed alternative instead of inferring them from the code alone.
- Stop unchanged retries; before a new, controlled attempt, confirm the configuration change and new mechanism, check suppression again, and compare the result.
Actions by owner
Sender
- Do not make further manual attempts without a change; give the administrator the complete response, attempt time, and account used without disclosing credentials.
Sender administrator
- Retain the complete response and inspect the client configuration, selected mechanism, account, and server endpoint for the specified attempt.
- Configure only a confirmed mechanism that complies with server policy, check suppression again, and make one controlled attempt instead of repeating the unchanged AUTH request.
Recipient administrator
- If you manage the server that returned the code, inspect its logs and the AUTH policy applied to the specified user and mechanism.
- Correct the policy only if it does not match the intended configuration; otherwise give the sender administrator safe information needed to select an allowed mechanism.
Provider
- If you operate an authentication service involved in the attempt, inspect its logs and policy for the specified time, user, and mechanism.
- Correct a confirmed problem in the managed layer or identify the permitted corrective path without requesting disclosure of credentials; do not trigger automatic suppression from the code alone.
Sources and verification
The canonical Tier-0 meaning of X.7.9, its relationship to the AUTH command, and the class-5 application of code 5.7.9 were verified against the IANA registry and RFC 2034, RFC 4954, and RFC 5248 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.7.9
- 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
- rfc4954T0 source
IANA registry reference for X.7.9
Last verified:
