451 domain not found (not relaying for this domain)What does 451 domain not found (not relaying for) mean?
The receiving server (or an upstream relay you sent through) cannot determine where to deliver the message because the recipient domain has no usable MX records, no fallback A records, or is not in the relay’s list of accepted domains. The 451 soft class allows retry, anticipating that the missing DNS records may appear (DNS propagation, recently transferred domain) or the relay configuration may be updated. In practice, this almost always indicates a configuration problem that needs to be fixed before delivery succeeds: either the recipient domain genuinely has no MX, or your relay is missing a transport route for that domain.
Is this a soft or hard bounce?
The MTA will retry for up to 5 days. Address the underlying DNS or relay configuration issue immediately to avoid wasted retries.
Common causes
Run dig MX recipient.com and dig A recipient.com. If both return nothing, the domain cannot receive mail at all.
Common cause: example.con instead of example.com. Verify the recipient address.
Your relay is configured to handle specific domains only. The recipient domain is not in the transport map. See 451 relay not permitted and 550 5.7.1 Relaying denied.
Recently registered or transferred domain with MX records not yet propagated. Resolves automatically within hours to 48 hours.
How to fix it
Run dig MX recipient.com and dig A recipient.com. Confirm at least one MX or A record exists for the recipient domain.
Common typos: .con instead of .com, .ne instead of .net, missing dots. Verify the recipient address with the sender.
If the error comes from your own relay (Postfix, Exim, or commercial), check the transport map or routes table. Add the recipient domain if missing.
If MX and A both return nothing after 48 hours, the domain is invalid and the address should be removed from your list.
Provider-specific notes
| MTA | Behavior |
|---|---|
| Postfix | Returns 451 when relay_domains or transport_maps have no entry for the recipient domain and recipient verification is enabled. |
| Exim | Returns 451 when no router accepts the address for delivery. Often paired with defer_unknown_users = true. |
| Sendmail | Returns 451 with not relaying for this domain in the message when the recipient domain is outside the local_domain_names set. |
Related error codes
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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
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