Gmail transport-security rejection

Gmail 550 5.7.29: This message was blocked because it wasn't sent over a TLS connection

Gmail rejected the bulk-sender message because the final SMTP connection to Gmail did not use TLS.

Problem description

The Gmail response is:

550 5.7.29 This message was blocked because it wasn't sent over a TLS connection.

Is this temporary or permanent?

SMTP 550 is permanent for this delivery attempt. Fix the outbound transport and send a new message.

Find the server that connected to Gmail

The final outbound server owns this failure. It may be your email provider, SMTP relay, security gateway, or self-managed mail server.

Collect:

the complete Gmail response, timestamp, and message ID;
the public IP and hostname that connected to Gmail;
the delivery log around EHLO, STARTTLS, and the TLS handshake;
every relay or gateway in the outbound path.

Fix the correct hop

For hosted email, send that evidence to the provider and ask why its Gmail connection did not use TLS. For a self-managed server, enable STARTTLS, use current TLS versions and trusted certificate authorities, and prevent plaintext fallback after a failed handshake.

Port 465 protects client submission. Gmail’s rejection concerns the separate server-to-server connection that finally delivered the message.

Verify the fix

Send a new message through the same route. Confirm a successful STARTTLS handshake in the outbound log and TLS on the relevant Received header. SPF, DKIM, DMARC, website HTTPS, and an inbound MX probe do not prove this transport is fixed.

Frequently asked questions

Does Gmail 5.7.29 refer to my website certificate?

No. It concerns the SMTP connection used to deliver this message to Gmail, not website HTTPS.

Can a DNS change enable SMTP TLS?

No. The server or relay making the final connection to Gmail must negotiate TLS successfully.

Is generic SMTP 5.7.29 the same error?

No. The IANA meaning is ARC validation failure. This page applies only when Gmail returns the exact TLS wording shown above.

Related email diagnostics

Gmail requirements for sending 5,000 or more messages per dayCheck Gmail bulk sender requirements for SPF, DKIM, DMARC alignment, PTR, TLS, spam rate and one-click unsubscribe using path-specific evidence. Google requirements for all senders to Gmail accountsUnderstand Google email sender requirements for SPF or DKIM, PTR and forward DNS, TLS, message format and spam rate before checking a sending domain and IP. SMTP 550 5.7.29: ARC validation failureUnderstand SMTP 550 5.7.29 ARC validation failure, find the first broken ARC set, and fix the intermediary that created it.

Primary sources: Gmail SMTP errors and codes, Google email sender guidelines.