SMTP Error 451: Local Error in Processing

Temporary failure Medium severity System RFC 5321

What it means

SMTP Error 451 means “Local Error in Processing.” 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.

At a glance

Code451

Bounce typeSoft (temporary)

SeverityMedium

CategorySystem

What to doQueued and retried automatically

StandardRFC 5321

What it looks like in your mail logs

451-4.3.0 Multiple destination domains per transaction is unsupported. Please try again. 451 4.3.0 b7-20020a05 - gsmtp

What does 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.

How 451 plays out

Your server attempts delivery

The recipient defers with a temporary 451 reply

Your server queues the message and retries on a back-off schedule

It delivers on a later attempt, or becomes a hard bounce if it keeps failing

Where 451 sits: soft vs hard bounce

	Soft bounce (4xx)	Hard bounce (5xx)
Nature	Temporary	Permanent
SMTP class	`4xx`	`5xx`
What to do	Let it retry	Suppress the address
Recoverable?	Often	No
451 is	✓ this code

Common causes of 451

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 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

What does SMTP error 451 mean?

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.

What is greylisting and how does it cause a 451 error?

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.

How long should I wait before retrying after a 451 error?

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.

Why does my mail server keep returning "451 temporary local problem"?

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.

Is a 451 SMTP error a permanent or temporary failure?

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.

Related bounce codes

Sources & further reading

RFC 5321 · The standard that defines this reply code Gmail SMTP errors and codes · Google Workspace reference covering 451 4.3.0 temporary rejections

Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes