523 5.7.10: commands pipelined after STARTTLS
Your client sent more commands straight after STARTTLS without waiting for the TLS handshake, which Gmail treats as a protocol violation.
- Gmail
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 523 5.7.10
- Provider
- Gmail and Google Workspace
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
523 5.7.10 SMTP protocol violation, no commands allowed to pipeline after STARTTLS.
What it means
With SMTP pipelining a client may send several commands without waiting for each reply. STARTTLS is the exception: it has to end a group, because anything sent after it, before the TLS handshake, travels in the clear and could have been injected by someone on the network path. Your client pipelined further commands after STARTTLS, so Gmail refused them. Google pairs the error with the unusual reply code 523 and refers to RFC 3207, the STARTTLS standard. The fix is in how the client handles STARTTLS.
Common causes
- A client that pipelines commands after STARTTLS.
- A buggy SMTP library or device firmware.
How to fix it
- 01Update the client or library to a version that waits for the TLS handshake after STARTTLS.
- 02If the client allows it, turn off pipelining.
- 03Relay devices that cannot be fixed through a mail server that handles STARTTLS correctly.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
Related codes
- 530 5.7.0STARTTLS required first.Gmail requires your client to switch the connection to TLS with STARTTLS before it signs in or sends mail, and your client did not.
- 501 5.7.11No parameters allowed.Gmail refused a command because your client added parameters that the command does not take, most likely to STARTTLS.
Further reading
All 108 Gmail codesEvery code in the wikiPaste your own bounce