Gmail 421 4.7.29
Rate limited: no TLS
Gmail 421 4.7.29 means: rate limited: no TLS. It is a temporary failure, so your server retries it. Gmail requires bulk senders to use TLS. Turn on STARTTLS for outbound connections to Gmail.
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
Your email has been rate limited because you're not using 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 | 421: <domain> Service not available, closing transmission channel (RFC) |
| Enhanced code | 4.7.29: ARC validation failure |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
In practice
Gmail requires bulk senders to use TLS for SMTP connections. This deferral means the connection was in plain text. Most mail servers support opportunistic STARTTLS, but it can be disabled in older configurations or broken by an expired certificate. Enable STARTTLS on outbound mail. If your server supports TLS but Gmail still reports none, check for an expired or self-signed certificate that makes the handshake fail and the connection fall back.
What the sender should do
Gmail requires bulk senders to use TLS. Turn on STARTTLS for outbound connections to Gmail.
Would verifying the list have prevented Gmail 421 4.7.29?
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 421 4.7.29 mean?
- Gmail 421 4.7.29 means: rate limited: no TLS. It is a temporary failure, so your server retries it. Gmail requires bulk senders to use TLS. Turn on STARTTLS for outbound connections to Gmail.
- Should I retry after Gmail 421 4.7.29?
- Your sending server retries it automatically, because it is a temporary (4xx) failure. Investigate only if it keeps coming back over several hours.
- Would verifying my list have prevented Gmail 421 4.7.29?
- 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