TL;DR
Permanent rejection: ARC validation failed on the message. Fix the forwarding or sealing chain before retrying. ARC failures in intermediated mail can block deliverability for forwarded traffic. Do not retry unchanged.
What this code means
ARC validation failed against the evaluating server's policy, and registry pattern X.7.29 can surface that outcome as code 5.7.29 in enhanced status class 5. The leading digit marks a permanent result in the register. The subcode places the failure in the ARC layer of the security family, apart from recipient-address validity and from naming any authentication check beyond ARC. Alone it does not specify which ARC set failed, whether sealing, keys, or hop order broke, or which administrator owns remediation of the chain.
Provider examples
550 5.7.29 This message was blocked because it wasn’t sent over a TLS connection. Gmail requires all bulk email senders to use TLS/SSL for SMTP connections. To set up TLS for email, visit TLS & SSL connections. To learn more about Gmail requirements for bulk email senders, visit Email sender guidelines. - gsmtpTechnical meaning
Failed ARC validation is the condition under which X.7.29 may be returned. Code 5.7.29 assigns the result to the permanent-failure class through its leading digit. The code does not identify the cause of the failed validation.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. The code confirms a problem with ARC validation, but it does not identify the precise condition or the party responsible for it.
- 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 ARC validation and suppression again.
Suppression decision
Operational guidance: do not automatically suppress the recipient address or domain based on 5.7.29 alone. Inspect the complete response, event scope, available ARC validation results, and delivery history, then base the suppression decision on the confirmed cause and applicable policy.
Common causes
- The message failed ARC validation; the code alone does not identify the precise condition that caused the failure.
Diagnostic steps
- Inspect the raw SMTP response or nondelivery report and confirm the exact 5.7.29 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, and system that returned the code.
- Use available ARC validation results, logs, and configuration to establish the actual failure condition; do not infer its cause or responsible party from the code alone.
- Stop unchanged retries. After a confirmed correction, check ARC validation 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 it with the intended message, and inspect available ARC validation results, logs, and configuration to establish the confirmed cause.
- Make only a confirmed correction, check ARC validation and suppression again, and verify the outcome with one controlled attempt.
Recipient administrator
- If you manage the system that returned the code, inspect its ARC validation logs and configuration 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 layer involved in ARC validation, 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
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 8617 — The Authenticated Received Chain (ARC) Protocol — ARC protocol for preserving authentication results across intermediaries.
- Gmail SMTP errors and codes — Official Gmail Help table of SMTP error messages and status codes.
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