451: sender domain could not be resolved
Yahoo could not confirm that the domain in your envelope sender or From address exists. As a 451 it is temporary; Yahoo can also return the same problem as a permanent 554.
- Yahoo
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 451
- Provider
- Yahoo and AOL
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
What it means
Yahoo checks that the domain to the right of the @ in the MAIL FROM address (RFC 5321) and in the From header (RFC 5322) is real. It does this with an SOA query, so a domain needs to answer for SOA even if it already has A or MX records. When the lookup times out or the DNS server fails, Yahoo answers with a temporary 451 and your server retries. When the answer is a definite failure, the same problem comes back as a permanent 554. This matters for subdomains used only for sending: Yahoo notes that when several subdomains appear in MAIL FROM or the From header, each needs SOA records set up as well as its A or MX records.
Common causes
- The domain in the MAIL FROM address or the From header has no SOA record, often a sending subdomain that only has A or MX records.
- The domain's name servers timed out or returned a server failure when Yahoo asked.
- The domain is misspelled, has lapsed, or was never registered.
How to fix it
- 01Look up the SOA record for the exact domain in your MAIL FROM and From addresses, including any subdomain, with dig or your DNS host's tools.
- 02If a sending subdomain has no SOA record, set one up as Yahoo's page asks, or send from a domain that has one.
- 03Check that every authoritative name server for the domain answers quickly and consistently.
- 04Once DNS answers correctly, let queued 451 messages retry, and resend any that came back as 554.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- SMTP Error Codes (Yahoo Sender Hub)senders.yahooinc.com
Related codes
- 421Temporary error or deferral.Yahoo or AOL refused your message for now with a 421, one of its two temporary error codes. Your server should keep it and retry; repeated deferrals point to reputation or complaints.
- 554Permanent rejection (554).Yahoo or AOL rejected the message for good with a 554. Yahoo uses this code for several problems, from failed authentication to unacceptable content, so the wording of the reply tells you which.
- 451Local error in processing.The receiving server hit a problem on its own side while handling the message and stopped. The reply is temporary, so the sending server will try again.
451 at other providers
- 451Temporary reject for misuse or guideline violations.GMX deferred your mail with a 451 because of an exceptional blocklist or a breach of its sending guidelines. The message is retried, but the cause has to be removed first.
- 451Transaction limit per session reached.Your server sent more messages over one SMTP session than GMX allows, currently 100, so GMX deferred the next one. Close the session and open a new one.
- 451Far more mail than expected.GMX received far more mail than it expected from your system, your DKIM domain or your network, and deferred the rest for now. A higher limit can be requested.
- 451Missing or unaligned DKIM under high traffic.GMX deferred your mail because it arrived in unusual volume without a DKIM signature that matches the From domain. Sign with an aligned DKIM domain to clear it.
- 451Low DKIM domain reputation.GMX deferred your mail because unusually high traffic arrived from a DKIM signing domain with a low reputation at GMX.
- 451Poor reputation of your network area.GMX deferred your mail because unexpected volume arrived from a network area with a negative reputation. An automatic check of your DKIM domain follows.
Further reading
All 7 Yahoo codesEvery code in the wikiPaste your own bounce