Gmail 421 4.7.27
Rate limited: SPF did not pass
Gmail 421 4.7.27 means: rate limited: SPF did not pass. It is a temporary failure, so your server retries it. Gmail requires bulk senders to pass SPF. Add your sending service to the SPF record of the envelope (MAIL FROM) domain.
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 SPF authentication didn't pass for this message. Gmail requires all bulk email senders to authenticate their email with SPF. Authentication results: SPF domain-name with IP address: ip-address = Did not pass. To set up SPF for your sending domains, visit Set up SPF. 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 | 4.7.27: Sender address has null MX |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
In practice
Gmail requires bulk senders, those sending around 5,000 messages a day to Gmail accounts, to pass SPF. This deferral includes the SPF domain and IP Gmail checked. The usual cause is a sending service that was never added to the SPF record of the envelope domain, or an SPF record that exceeds 10 DNS lookups and returns permerror.
What the sender should do
Gmail requires bulk senders to pass SPF. Add your sending service to the SPF record of the envelope (MAIL FROM) domain.
Would verifying the list have prevented Gmail 421 4.7.27?
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 4.7.27 mean?
- Gmail 421 4.7.27 means: rate limited: SPF did not pass. It is a temporary failure, so your server retries it. Gmail requires bulk senders to pass SPF. Add your sending service to the SPF record of the envelope (MAIL FROM) domain.
- Should I retry after Gmail 421 4.7.27?
- 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 4.7.27?
- 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