SMTP enhanced status code 5.7.29

What the registry says

The basic codes it travels with

• 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)

In practice

In the registry, X.7.29 is an ARC validation failure, which matters for forwarded mail and mailing lists. Gmail uses 421 4.7.29 and 550 5.7.29 for bulk mail sent without TLS. Check the text: TLS is the more common reason in Gmail bounces.

What the sender should do

For ARC, check the forwarders in the path. For Gmail's use, enable TLS on the sending connection.

How providers use X.7.29

Quoted from each provider’s own error reference.

• Gmail 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. To set up TLS for email, visit TLS & SSL connections. To learn more about Gmail requirements for bulk email senders, visit Email sender guidelines.

• Gmail 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.

Common questions

What does 5.7.29 mean?

X.7.29 means the message failed ARC validation, the chain that records authentication results across forwarders. Gmail also uses 5.7.29 for bulk mail sent without TLS.

What should I do about 5.7.29?

For ARC, check the forwarders in the path. For Gmail's use, enable TLS on the sending connection.

Would an email verifier have caught 5.7.29?

No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.