451 Grey bounce: SPF failure or not definedWhat does 451 Grey bounce SPF failure mean?
The receiving server combines a greylisting policy with SPF validation: messages from senders without a valid SPF record (or with SPF that fails) are held in a grey queue for extended time rather than accepted immediately or rejected hard. The 451 soft response signals this hybrid policy: I’ll keep your message but I will not deliver until you fix authentication and resend. Common at smaller hosting providers and some European MTAs running custom Postfix or Exim setups. The fix is to configure a valid SPF record and ensure your sending IP is authorized in the SPF policy.
Is this a soft or hard bounce?
Unlike standard greylisting (which clears on retry), this variant requires SPF to be fixed before retries succeed. Fix SPF first, then queue retries deliver.
Common causes
Sending domain has no SPF TXT record. Receiver treats absence as authentication failure under strict policy.
SPF record published but your sending IP is not in the authorized list. Common after migrating to a new sending IP without updating SPF.
Malformed SPF (e.g. multiple SPF records on the same domain, syntax errors) causes evaluators to return permerror, treated as fail.
Nested includes pushing past RFC 7208’s 10 DNS lookup limit. Returns permerror.
How to fix it
Run dig TXT yourdomain.com and look for a TXT record starting with v=spf1. If missing, see SPF guide for setup.
Use kitterman.com SPF validator or similar. Enter your domain and sending IP; result should be PASS.
Add your sending IP or include your ESP’s mechanism. Use SPF generator if you need a fresh record.
DNS changes propagate in minutes to hours. Once SPF passes, the grey queue retries will deliver automatically.
Provider-specific notes
| SPF result | Behavior |
|---|---|
| No SPF record | Receiver treats as fail under strict greylisting policy. 451 returned. |
| SPF softfail (~all) | Some strict MTAs treat softfail as fail. Switch to -all if your record is complete. |
| SPF fail (-all, IP not listed) | Definitive authentication failure. Add sending IP or includes. |
| SPF permerror (syntax / lookup limit) | Treated as fail. Audit record for syntax errors and lookup count. |
Related error codes
About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
About SMTPedia
SMTPedia is an independent email industry reference covering SMTP, IMAP, POP3, email deliverability, marketing platforms, DNS authentication, and email verification. Every article is researched from official provider documentation, IETF RFCs, and industry best practices. Settings and configurations are verified quarterly.
We are cited as a source by ChatGPT, Microsoft Copilot, and thousands of email professionals worldwide. Learn more about our editorial process.
