451: local 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.
- Standard
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 451
- Provider
- Standard SMTP codes
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
451 Requested action aborted: local error in processing
What it means
A 451 says the server ran into a problem and aborted, describing it as local. It is one of the most frequent temporary replies, and servers use it for many things: internal errors, greylisting, rate limits, and temporary DNS failures while checking the sender (RFC 7208 recommends 451 when an SPF check hits a temporary error). RFC 5321 also tells clients to treat a connection that drops mid-transaction as if a 451 had been received. The sending server keeps the message and retries, and only returns it if every attempt fails until its retry period ends.
Common causes
- A temporary internal error on the receiving server.
- Greylisting of a sender the server has not seen before.
- A temporary DNS failure while the receiver checked the sender's SPF record or domain.
- A connection that dropped in the middle of the transaction.
How to fix it
- 01Let the sending server retry.
- 02Read the enhanced code and text for the cause.
- 03If the text mentions SPF or DNS, check that your domain's DNS servers answer reliably.
- 04If it persists for one domain only, contact that domain's administrator.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 5321, section 4.2.3: Reply Codes in Numeric Orderrfc-editor.org
- RFC 5321, section 3.8: Terminating Sessions and Connectionsrfc-editor.org
- RFC 7208, section 8.6: Temperror (SPF)rfc-editor.org
Related codes
- 450Mailbox temporarily unavailable.The server did not take the message for this mailbox for now, because the mailbox is busy or temporarily blocked by policy. The sending server will try again.
- 421Service not available.The receiving server is closing the connection and will not take mail right now. The reply is temporary, so the sending server keeps the message and tries again later.
- 4.4.1No answer from the receiving server.The sending server tried to connect to the receiving server and got no answer, so it keeps the message and tries again later.
- 4.4.3DNS or directory lookup failed.The message could not be passed on because a directory server, most often DNS, did not answer; the lookup and the delivery are retried.
- 4.4.5Mail system congested.The mail system was too busy to deliver the message at that moment, so delivery is delayed and will be tried again.
451 at other providers
- 451Sender 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.
- 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.
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce