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Standard SMTP codes · permanent error

5.7.29: ARC validation failed

The message carried an ARC chain, the record that forwarders add when they pass mail on, and the receiver found that chain invalid.

Standard
Provider
Permanent (5xx)
Class
Sources checked
By , email deliverability consultant
Author

At a glance

Code
5.7.29
Temporary or permanent
PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
Who has to act
Either side, depending on the cause. Start with the checks below that are yours.
What the server says
5.7.29 ARC validation failure

What it means

ARC (RFC 8617) lets forwarders such as mailing lists record the authentication results they saw, so a later receiver can trust a message that SPF or DKIM no longer validate after forwarding. If a receiver's ARC validation fails, RFC 8617 says it may signal this with 5.7.29 and the matching basic reply code. Because ARC failures usually come together with other authentication failures, the RFC also allows the more general 5.7.26 instead. The original sender has little control over ARC; the forwarding service that sealed the message does.

Common causes

  • A forwarder or mailing list added a broken or invalid ARC seal.
  • The message was changed after an ARC set was added.

How to fix it

  1. 01If you run the forwarder or list, check its ARC signing keys and configuration.
  2. 02Make sure the original message passes SPF, DKIM and DMARC before it is forwarded.
  3. 03If you are a subscriber of the list, report the bounce to its operator.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

  1. 550 5.7.26Multiple authentication checks failed.The message failed more than one authentication check, for example SPF and DKIM, and the receiving server's policy rejects it.StandardPermanent
  2. 550 5.7.20No valid DKIM signature.The receiving server requires a valid DKIM signature and found none that passed verification, so it refused the message.StandardPermanent

5.7.29 at other providers

  1. 550 5.7.29Not sent over TLS, blocked.Gmail rejected the message because your server delivered it without TLS encryption, which Google's sender guidelines require.GmailPermanent

All 105 standard codesEvery code in the wikiPaste your own bounce

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