Gmail 523 5.7.10
No pipelining after STARTTLS
Gmail 523 5.7.10 means: no pipelining after STARTTLS. It is a permanent failure, so do not retry it unchanged. The client pipelined commands after STARTTLS. Wait for the TLS handshake to finish before sending more commands.
Gmail’s exact wording
Quoted from Google Workspace Admin Help: Gmail SMTP errors and codes, checked 2026-11-05. Placeholders are Gmail’s own.
Reply text
SMTP protocol violation, no commands allowed to pipeline after STARTTLS. For more information, go to About SMTP error messages and review RFC 3207 specifications.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 523 |
| Enhanced code | 5.7.10: Encryption Needed |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
After STARTTLS, the client must wait for the TLS handshake and send EHLO again before any other command. Some hand-written clients pipeline commands straight after STARTTLS, which Gmail rejects. Standard mail libraries handle this correctly. It affects every message from that client, regardless of recipient.
What the sender should do
The client pipelined commands after STARTTLS. Wait for the TLS handshake to finish before sending more commands.
Would verifying the list have prevented Gmail 523 5.7.10?
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.
Common questions
- What does Gmail 523 5.7.10 mean?
- Gmail 523 5.7.10 means: no pipelining after STARTTLS. It is a permanent failure, so do not retry it unchanged. The client pipelined commands after STARTTLS. Wait for the TLS handshake to finish before sending more commands.
- Should I retry after Gmail 523 5.7.10?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. The client pipelined commands after STARTTLS. Wait for the TLS handshake to finish before sending more commands.
- Would verifying my list have prevented Gmail 523 5.7.10?
- 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.
Sources
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05