MailFixIt

Gmail temporary SPF deferral

Gmail 421 4.7.27: Your email has been rate limited because SPF authentication didn't pass

Gmail uses this temporary response when a bulk-sender message does not pass SPF and the sending path is rate limited. Keep normal queue retries with backoff, but investigate the reported IP and envelope MAIL FROM domain before sending more traffic.

Problem description

A representative Gmail response includes the evaluated domain and IP:

421 4.7.27 Your email has been rate limited because SPF authentication didn't pass for this message. Authentication results: SPF example.com with IP address: 192.0.2.10 = Did not pass.

Run this automated test on my domain

Enter your domain. MailFixIt checks the relevant DNS records automatically and shows the evidence.
Operational priority: let the mail queue handle retries; avoid manual retry loops or traffic bursts. First verify whether the stated IP is an intended sender for the stated envelope domain.
Is this temporary or permanent?

421 is temporary, so a standards-compliant mail queue should retry with backoff. Retrying does not repair a persistent SPF authorization problem.

What MailFixIt verifies

With the sending IP and envelope MAIL FROM domain, the diagnostic evaluates the current supported SPF path as pass, fail/softfail, or unknown. It does not claim to reproduce Gmail’s rate limit, sender reputation, historical DNS view, or later acceptance decision.

Fix the SPF path without widening access

  1. Copy the complete Gmail response and preserve the queue or message ID.
  2. Confirm that the reported IP belongs to an intended sender or relay.
  3. Evaluate SPF at the exact envelope domain, including any return-path subdomain.
  4. If the path should be authorized, apply the sending provider’s current documented SPF value while preserving other legitimate senders.
  5. If the IP is unexpected, correct the route, relay, or credentials instead of authorizing it.
  6. Check for multiple SPF records, broken dependencies, and DNS lookup-limit failures.

Confirm recovery

Let the existing queue retry with normal backoff after corrected DNS is visible. Send one new message through the same route and inspect its Authentication-Results. An SPF pass confirms the repair, but does not guarantee inbox placement.

Frequently asked questions

What does Gmail 421 4.7.27 mean?

Gmail temporarily deferred a bulk-sender message because SPF did not pass for the reported envelope domain and sending IP.

How is 421 4.7.27 different from 550 5.7.27?

421 is a temporary response that queues should retry with backoff. 550 permanently rejects that delivery attempt. Both still require investigation when the same SPF path keeps failing.

Will retrying fix the SPF problem?

No. A later retry may be accepted only if the underlying condition was transient. A persistent authorization error remains until the sender path or SPF policy is corrected.

Should I authorize the IP shown by Gmail?

Only after confirming it belongs to an intended sender. If it is unexpected, investigate the relay or account instead of widening SPF.

Related email diagnostics

Gmail 550 5.7.27: This message didn't pass SPF authenticationDiagnose Gmail 550 5.7.27 using the actual sending IP and envelope MAIL FROM domain, then correct only the SPF or sender path that failed. SPF is not configured for the envelope sender domainDiagnose SPF missing on a bounce or MAIL FROM subdomain. Check the exact envelope domain, add only authorized senders, and verify safely. SMTP 550 5.7.23: SPF validation failedUnderstand enhanced SMTP status 5.7.23 SPF validation failed, identify the sender IP and MAIL FROM domain, and check the relevant SPF configuration. Gmail requirements for sending 5,000 or more messages per dayCheck Gmail bulk sender requirements for SPF, DKIM, DMARC alignment, PTR, TLS, spam rate and one-click unsubscribe using path-specific evidence.

Run this automated test on my domain

Enter your domain. MailFixIt checks the relevant DNS records automatically and shows the evidence.

Primary sources: Gmail SMTP errors and codes, Google SPF setup guidance, Google email sender guidelines, RFC 7208 SPF.