MailFixIt

Gmail SMTP delivery response

Understand Gmail 523 5.7.10: Invalid SMTP pipelining after STARTTLS

The sending SMTP client issued another command before STARTTLS negotiation had completed.

Problem description

A representative response is:

523 5.7.10 Invalid SMTP pipelining after STARTTLS.
Is this temporary or permanent?

Retrying the same client behavior will fail again. Correct or update the SMTP client before retrying.

What this response means

Gmail accepted the STARTTLS request, but the sending client continued to pipeline SMTP commands. After Gmail replies that TLS may begin, the client must complete the TLS handshake and start the encrypted SMTP session with a new EHLO.

Fix it

  1. Update the SMTP library, mail transfer agent, appliance, or firmware that opened the connection.
  2. Do not send AUTH, MAIL FROM, RCPT TO, or any other SMTP command together with or immediately after STARTTLS.
  3. Wait for the server’s 220 response, complete the TLS handshake, then send a new EHLO.
  4. Enable PIPELINING only for commands advertised in the post-TLS EHLO response.
  5. Capture one new SMTP transcript to confirm the corrected order.

This is a sending-software error. DNS changes and repeated delivery attempts cannot repair it.

Who can fix it

The owner of the sending application or outbound SMTP server must change the client behavior. If a hosted relay generated the bad sequence, send its support team the complete response and timestamp.

Use the related guides for Gmail sender requirements; they do not replace correction of this SMTP protocol violation.

Frequently asked questions

What does Gmail 523 5.7.10: Invalid SMTP pipelining after STARTTLS mean?

The SMTP client crossed the STARTTLS boundary incorrectly by sending another SMTP command before completing TLS and issuing a new EHLO.

Can MailFixIt prove this entire bounce from public DNS?

No. The current checker can evaluate selected public DNS configuration, but it cannot recreate recipient state, credentials, reputation, message content, or a historical provider SMTP decision.

Related email diagnostics

Gmail 550 5.7.26: This email has been blocked because the sender is unauthenticatedDiagnose Gmail 550 5.7.26 safely. Separate unauthenticated-mail, SPF hard-fail and DMARC-policy branches before changing email DNS. 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.

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