Enhanced SMTP status code · RFC 3463 · IANA
5.7.29 ARC validation failure
The message failed ARC validation (RFC 8617), the chain that keeps authentication results across forwarding.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
Also seen as:
4.7.29 - Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
- Associated basic codes (IANA registry)
How 5.7.29 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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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.
The message did not come over a TLS connection, which Gmail requires from bulk senders. Check with Mail delivery test.
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421 4.7.29
Your email has been rate limited because you're not using a TLS connection. Gmail requires all bulk email senders to use TLS/SSL for SMTP connections.
The message did not come over a TLS connection, which Gmail requires from bulk senders. Check with Mail delivery test.
What you can check here
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026