MailFixIt

Gmail temporary authentication deferral

Gmail 451 4.7.26: Unauthenticated email is not accepted due to DMARC policy, but temporary DNS failures prevent authentication

Gmail could not authenticate the message because a temporary DNS failure occurred while the From domain's DMARC policy required authentication.

Problem description

The Gmail response contains:

451 4.7.26 Unauthenticated email from domain-name is not accepted due to domain's DMARC policy, but temporary DNS failures prevent authentication.

Run this automated test on my domain

Enter your domain. MailFixIt checks the relevant DNS records automatically and shows the evidence.
Is this temporary or permanent?

SMTP 451 is temporary. The sending server should retry with backoff while you check repeated DNS or authentication failures.

Fix the repeated deferral

  1. Save the complete response and timestamp.
  2. From the same message, identify the visible From domain, envelope domain, DKIM d= domain, and selector.
  3. Check DMARC at the From domain and its organizational-domain fallback.
  4. Check the exact SPF and DKIM DNS names used by the message.
  5. If DNS answers vary, inspect authoritative nameserver health, delegation, DNSSEC, and authentication dependencies.
  6. Correct a confirmed problem, then let the queue retry or send a new test.

Keep the DMARC policy

Do not weaken a correct policy to hide unreliable DNS or an unauthorized sender. Restore one reliable aligned SPF or DKIM path.

Frequently asked questions

Should I change DMARC from reject to none?

No. First check DNS reliability and make the legitimate message pass aligned SPF or DKIM.

Will Gmail retry?

The sending server should retry a 451 response. Repeated failures need investigation rather than faster retries.

Related email diagnostics

Gmail 550 5.7.26: This email has been blocked because the sender is unauthenticatedDiagnose Gmail 550 5.7.26 safely. Separate unauthenticated-mail, SPF hard-fail and DMARC-policy branches before changing email DNS. Gmail 550 5.7.40: Your message was blocked because the sending domain doesn't have a DMARC record or policyDiagnose Gmail 550 5.7.40 with a current RFC 9989 DMARC policy check. Distinguish a missing or invalid policy from temporary DNS failures and message alignment. DMARC policy for the visible From subdomain is unexpectedFind the effective DMARC policy for a visible From subdomain. See whether p, sp, np or test mode applies before changing DMARC DNS.

Primary sources: Gmail SMTP errors and codes, RFC 9989: Domain-based Message Authentication, Reporting, and Conformance.