TL;DR
Permanent rejection because a security feature in the message could not be handled on the delivery path. Align protocol support or adjust the feature before retrying. Unresolved path gaps can affect deliverability if the same send keeps failing. Do not retry unchanged.
What this code means
Security features in the message, such as secure authentication, could not be supported on the delivery protocol along the path; code 5.7.4 reports that under pattern X.7.4. In the enhanced status register this unsupported-feature detail is permanent, and the registry defines X.7.4 for permanent use only. The numeric reply points to a protocol or configuration gap on the delivery path, not to unauthorized sender identity or an invalid recipient address, and it alone does not name the exact feature or the system that lacked support.
Technical meaning
A message that contained security features, such as secure authentication, that could not be supported on the delivery protocol falls under X.7.4. The registry description says this detail is useful only as a permanent error, and code 5.7.4 applies it in class 5.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. The code alone identifies neither the exact unsupported feature nor the system on the path that could not support it, and it does not 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 of the same unchanged attempt. Consider a new send only after a verified change to the security feature, protocol support, or delivery path; check suppression again before the attempt.
Suppression decision
Operational guidance: do not automatically add an address or domain to a suppression list based on 5.7.4 alone. Inspect the complete response, the security features used, systems on the path, and event history, then make the suppression decision according to the established cause and applicable policy.
Common causes
- The message used a security feature, such as secure authentication, that could not be supported on the delivery protocol.
Diagnostic steps
- Inspect the raw SMTP response or nondelivery report and confirm that the enhanced code is exactly 5.7.4 and that the basic reply is in the 5xx class; retain the complete response text.
- Correlate the event with the intended message, attempt time, transaction stage, and systems on the path; use available logs and configuration to identify the security feature and where it could not be supported instead of inferring those details from the code alone.
- Stop unchanged retries; before a controlled new send, verify a material correction to the feature, configuration, or path and reassess suppression.
Actions by owner
Sender
- Do not resend the same unchanged message; give the administrator the complete response and attempt context.
- Do not disable or weaken a security feature based on the code alone; apply only a verified change prepared by the appropriate administrator.
Sender administrator
- Retain the complete response and attempt context, then inspect the message's security features, client configuration, and protocol support on the known path.
- Correct only a confirmed incompatibility or select a path that supports the required feature; check suppression again before a controlled new attempt.
Recipient administrator
- If the code came from a recipient system you manage, inspect its logs, protocol configuration, and supported security features for the specified attempt.
- Correct the confirmed lack of support within your authority if the feature should be supported, or give the sender administrator exact, safe context for the refusal.
Provider
- For the specified attempt, inspect managed-service logs and identify the security feature and protocol segment where it could not be supported.
- Correct a confirmed problem in the managed layer or identify the owner of the required correction; do not trigger automatic suppression from 5.7.4 alone.
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 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
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