The receiving server encountered a temporary internal error while processing your email. This is typically a server-side issue such as database failures, DNS lookup timeouts, or anti-spam processing errors that will resolve on retry.
What Does Error 451 Mean?
SMTP code 451 indicates the receiving server experienced an internal error while trying to process your email. The error is on the receiving server's side, not yours. Common causes include temporary DNS lookup failures during SPF/DKIM verification, database connectivity issues, anti-spam engine errors, or resource exhaustion.
Gmail may return 451 4.3.0 during temporary processing issues. Microsoft 365 uses 451 for various transient failures including authentication checking delays. Most 451 errors resolve within minutes to hours as the receiving server's issue clears up. Your MTA should retry automatically.
If you see 451 consistently from a specific provider, it could indicate that the provider is having difficulty verifying your authentication records (SPF, DKIM, DMARC). Check that your DNS records are correctly configured and that your DNS provider is responsive.
Common Causes
- Temporary DNS lookup failure during SPF or DKIM verification on the receiving side
- Anti-spam or anti-virus engine encountered a temporary error
- Database connectivity issue on the receiving server
- Server resource exhaustion (CPU, memory) during message processing
- Backend authentication service timeout
How to Fix Error 451
- Wait for automatic retry - most 451 errors resolve within minutes
- If persistent, verify your SPF and DKIM DNS records are correctly published and resolvable
- Check that your DNS provider is not experiencing outages
- Contact the recipient postmaster if 451 errors persist for more than 24 hours
Frequently Asked Questions
SMTP error 451 means "Temporary local problem -- please try later," indicating the receiving mail server encountered a temporary issue and cannot process your email right now. This is a soft bounce (4xx-class error), meaning the server expects the problem to resolve and delivery to succeed on a subsequent attempt. Common causes include server overload, rate limiting, DNS resolution failures, greylisting, or anti-spam services (like SpamAssassin or ClamAV) being temporarily offline.
Greylisting is an anti-spam technique where the recipient's mail server intentionally rejects the first delivery attempt from unknown senders with a temporary 451 error, then accepts the message when the sender's server retries after a delay. Legitimate mail servers will automatically retry (typically within 5-15 minutes), while spam bots usually do not. The initial delay caused by greylisting is typically 1-15 minutes, but once your sender/recipient/IP combination is whitelisted, subsequent emails are delivered without delay.
Most properly configured mail servers will automatically retry delivery after a 451 error using exponential backoff, starting at about 5-15 minutes and increasing with each attempt. If you are manually resending, wait at least 15-30 minutes. For greylisting specifically, the SMTP standard recommends a minimum retry interval of 30 minutes, though most receiving servers accept retries much sooner. If the 451 error persists for more than a few hours, the issue may require the recipient's server administrator to investigate.
Persistent "451 temporary local problem" errors usually point to a configuration issue on the receiving server, such as disabled or crashed anti-virus/anti-spam services (ClamAV, SpamAssassin), misconfigured MX records, DNS resolution failures, or the server running out of resources. If you are the server administrator, check that all mail-processing services are running, verify MX and DNS records are correct, and ensure the server has adequate disk space and memory. If you are the sender, contact the recipient or their postmaster to report the ongoing issue.
SMTP error 451 is a temporary failure (soft bounce). The 4xx class of SMTP response codes specifically indicates transient conditions that the server expects to clear up, unlike 5xx codes which are permanent failures. Your mail server will queue the message and retry delivery automatically, typically for up to 4-5 days before generating a permanent non-delivery report (NDR). No immediate action is usually required from the sender, as the email will likely be delivered once the temporary condition resolves.