SMTP code 451 indicates that the mail server experienced an internal or local error that prevented it from completing the requested action. This is a catch-all temporary error — the server's storage backend may have failed, a content filter may have timed out, a DNS lookup required for policy checks may have failed, or the server ran into an unexpected condition. The key distinction is that the error is on the receiving server's side, not the sender's. The sending MTA should queue the message and retry. If 451 persists across multiple retries over several hours, there is likely a systematic problem on the receiving end.
Many mail servers pass messages through content filters (SpamAssassin, ClamAV, milters) before accepting them. If the filter process crashes, takes too long, or is unreachable, the server returns 451 rather than accepting an unscanned message or silently dropping it.
The server attempted to look up SPF, DKIM, DMARC, or reverse DNS records for the sender but the DNS query failed or timed out. Since these checks are required by the server's policy, it returns 451 to defer the message until DNS is working again.
The mail server queried its user database, alias table, or LDAP directory and received an error or timeout. Without the ability to verify routing, it defers with 451.
The server attempted to write the incoming message to its spool directory but encountered a disk error, permission issue, or filesystem corruption. This prevents the message from being stored and triggers a 451.
451 is temporary. Your MTA should automatically retry. If 451 persists for more than 4-6 hours, the receiving server likely has a sustained infrastructure problem.
If the receiving server is failing on SPF/DKIM/DMARC lookups, ensure your domain's DNS records are correctly published and resolvable from multiple locations.
dig TXT example.com +shortCheck DNS Records
A malformed SPF record can cause DNS parsing errors on the receiving server, leading to 451. Use a DNS lookup tool to check your SPF record for syntax issues, too many includes (10-lookup limit), or missing mechanisms.
Check DNS RecordsIf retries fail for more than 24 hours, email [email protected] or check the domain's abuse/postmaster page. Include the full SMTP error response and timestamps.
If you recently updated SPF, DKIM, or DMARC records, verify that the changes have propagated to all DNS servers. Partial propagation can cause intermittent 451 errors.
Check PropagationThe SMTP server is temporarily unavailable, closing the connection. The sending server should retry later.
The recipient's mailbox is temporarily unavailable. The server will not process the command right now but the sender should retry.
The server does not have enough storage to process the command. The sender should retry later when space may be available.