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SMTP error code · Standard (RFC)

SMTP 451: requested action aborted, local error in processing

451 Temporary failure (4xx): the sender retries
By iBlockSpam Team Checked against sources 3 sources How we check our content
What it means

The receiving server ran into a temporary problem while handling the message and aborted it. The sender should retry; most 451 errors clear on their own.

What the bounce says
451 4.3.0 Email server has temporarily rejected this message.

Gmail's generic 451 text, as listed in Google's SMTP errors reference.

What 451 means

RFC 5321 lists 451 as "Requested action aborted: local error in processing". "Local" means the problem is on the receiving side: a failed lookup, a busy content scanner, a database that didn't answer in time. Like every 4xx reply, it is temporary, so the message stays in your queue and your server retries.

In practice the enhanced code narrows it down. Gmail documents 451 4.4.2 for a timeout, 451 4.7.23 for a sending IP without a matching PTR record, and 451 4.7.26 when a DMARC check couldn't finish because of temporary DNS failures. Yahoo says it may return 451 when it can't resolve the domain in your MAIL FROM or From header because of a timeout or DNS server failure.

That last group matters: some 451 errors are caused by the sender's DNS. If your domain's name servers are slow or failing, receivers can't check SPF, DKIM or DMARC, and they defer your mail until they can.

Likely causes

  • A temporary fault on the receiving server, such as a timeout or a busy scanner.
  • The receiver couldn't look up your domain's DNS records (SPF, DKIM, DMARC or the domain itself).
  • Your sending IP has no PTR record or one that doesn't match, and the receiver is rate limiting you.
  • Greylisting or another temporary policy check.

How to fix it

If you sent the message

  1. Let your server retry. Most 451 errors clear within an hour.
  2. If the text mentions DNS, SPF or DMARC, check that your domain's name servers answer and that your records are valid.
  3. Confirm your sending IP has a PTR record that resolves back to the same address.

If you're the recipient or their admin

  1. If your server sends 451 to many senders, check its logs for timeouts in DNS lookups, antivirus or content filtering.
Email Health Check SPF Checker Reverse DNS (PTR) Lookup

Frequently asked questions

Is a 451 error my fault or the recipient's?

Often neither: it's a temporary fault on the receiving side. But if the text mentions DNS, SPF, DMARC or PTR records, the receiver couldn't verify your setup, and the fix is on your side.

How long will my server keep retrying after a 451?

That depends on your mail server's queue settings. RFC 5321 recommends retrying at intervals of at least 30 minutes and not giving up for at least four to five days.

Sources

  1. RFC 5321: Simple Mail Transfer Protocol (reply codes, section 4.2)
  2. Gmail SMTP errors and codes (Google Workspace Admin Help)
  3. SMTP Error Codes (Yahoo Sender Hub)

Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.

Related codes

Related guides

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