Gmail 550 5.7.29
Blocked: no TLS
Gmail 550 5.7.29 means: blocked: no TLS. It is a permanent failure, so do not retry it unchanged. Gmail requires bulk senders to use TLS. Enable STARTTLS on outbound connections.
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
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.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 550: Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons) (RFC) |
| Enhanced code | 5.7.29: ARC validation failure |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
The permanent form of the TLS requirement. Modern servers and sending services use TLS by default, so this usually points to an old self-hosted server, a device sending direct to Gmail's MX, or a TLS negotiation that fails and falls back to plain text. Check the certificate expiry date as well, since a failed handshake can make a server fall back to plain text.
What the sender should do
Gmail requires bulk senders to use TLS. Enable STARTTLS on outbound connections.
Would verifying the list have prevented Gmail 550 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.
Common questions
- What does Gmail 550 5.7.29 mean?
- Gmail 550 5.7.29 means: blocked: no TLS. It is a permanent failure, so do not retry it unchanged. Gmail requires bulk senders to use TLS. Enable STARTTLS on outbound connections.
- Should I retry after Gmail 550 5.7.29?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. Gmail requires bulk senders to use TLS. Enable STARTTLS on outbound connections.
- Would verifying my list have prevented Gmail 550 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.
Sources
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (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