421 4.7.27 Your email has been rate limited because SPF authentication didn't pass for this message
Gmail deferred or blocked mail that failed SPF
Seen in Gmail, Google Workspace (550 5.7.27 when permanent)
Gmail requires bulk senders to pass SPF; this message failed SPF, so it was rate limited (421 4.7.27) or blocked (550 5.7.27).
Common causes
- The sending IP is not in the SPF record of the envelope sender (Return-Path) domain.
- The SPF record has errors or exceeds the 10 DNS lookup limit.
- A new ESP or server was added without updating SPF.
How to fix it
- Add the sending service's include or IP to SPF.
- Keep SPF under 10 lookups and publish only one SPF record per domain.
- Check which domain is in the Return-Path; SPF is checked against that domain, not the From address.
Got a different message? Paste it into the error explainer.
Related errors
- 550 5.7.26 Unauthenticated email is not accepted · Gmail rejected mail that failed DMARC
- 550 5.7.1 Relaying denied / Message rejected · The receiving server refused the message on policy grounds
- 421 4.7.0 Try again later · Temporarily deferred by the receiving server
- 550 5.7.515 Access denied, sending domain does not meet the required authentication level · Outlook.com rejected unauthenticated bulk mail
- 550 5.7.23 SPF validation failed · The message failed the SPF check
- 550 5.7.509 Access denied, sending domain does not pass DMARC verification and has a DMARC policy of reject · Microsoft 365 rejected mail that failed DMARC
- 421 4.7.28 Our system has detected an unusual rate of unsolicited mail originating from your IP address · Gmail is rate limiting mail from your IP
- 550 5.7.1 Our system has detected that this message is likely unsolicited mail · Gmail blocked the message as spam
- 421 4.7.26 This mail has been rate limited because it is unauthenticated · Gmail deferred mail that passed neither SPF nor DKIM
- 421 4.7.0 [TSS04] Messages from x.x.x.x temporarily deferred due to unexpected volume or user complaints · Yahoo is temporarily deferring your mail
- 550 5.7.25 The IP address sending this message does not have a PTR record · The sending IP has no reverse DNS
- 550 5.1.1 User unknown · The recipient address doesn't exist