What causes a 451 4.3.0 'Mail server temporarily rejected message' error, and how can it be resolved?
Matthew Whittaker
Co-founder & CTO, Suped
Published 18 Jun 2025
Updated 18 Aug 2025
9 min read
Encountering a 451 4.3.0 'Mail server temporarily rejected message' bounce error can be a frustrating experience for anyone involved in email sending. This message indicates that the recipient's mail server temporarily refused to accept your email, but it implies that the situation is transient and the message should be retried later. While it's not a permanent failure, it can certainly delay critical communications and impact your deliverability rates if not understood and addressed.
Unlike hard bounces (5xx errors) that signal a permanent delivery failure, a 451 error is considered a soft bounce. Mail Transfer Agents (MTAs) are designed to automatically re-attempt delivery of messages that receive a 4xx response code. However, relying solely on retries without understanding the underlying cause can lead to prolonged delays, eventual permanent failures, and negatively affect your sender reputation.
My aim with this guide is to demystify the 451 4.3.0 error. I will explore what this specific SMTP code means, delve into the most common causes, and provide practical steps for diagnosing and resolving the issue, whether you are the sender or responsible for the receiving mail server. Understanding these nuances is crucial for maintaining optimal email deliverability.
Decoding the 451 4.3.0 SMTP response
The 451 status code is part of the SMTP (Simple Mail Transfer Protocol) reply codes, which are used by mail servers to communicate the status of an email transaction. When a sending server receives a 451 response, it means the receiving server encountered a temporary problem that prevented it from completing the request. The key takeaway here is temporary. This implies that the mail server expects the sending server to try again later.
The 4.3.0 portion of the error code provides a more specific detail about the nature of the temporary failure. In this case, 4.3.0 typically signifies a Mail system status or Internal mail system error. This often points to a problem within the receiving server's infrastructure, such as a temporary software glitch, resource exhaustion, or a local policy violation that prevents immediate delivery. For more details on this specific error, you can refer to Gmail's documentation on SMTP errors.
The distinction between a 4xx error and a 5xx error is critical. A 5xx error, like a 550 permanent failure, signifies that the email will never be delivered to the recipient for a variety of reasons, such as an invalid address or being blocklisted. With a 451, the hope for successful delivery remains, provided the transient issue is resolved by the receiving server or the sending server adjusts its behavior based on implied policy. The SMTP Field Manual classifies 451 as a Transient Negative Completion Reply.
Common culprits behind the temporary rejection
Several factors can lead to a 451 4.3.0 error. Many of these issues stem from temporary conditions on the receiving mail server, but some can also be triggered by the sending server's behavior or configuration. Identifying the specific cause is the first step toward resolution.
The most common reason for a 451 4.3.0 error is resource exhaustion or overload on the recipient's mail server. This could mean the server is experiencing high traffic, has run out of disk space, is low on memory, or its CPU is maxed out. In such scenarios, the server temporarily rejects incoming mail to prevent a complete crash or data loss. The intention is to signal I can't take your mail right now, but try again later.
Another frequent cause involves rate limiting or throttling policies implemented by the receiving mail server. If you send a high volume of emails from a particular IP address or domain within a short period, the recipient server might interpret this as suspicious behavior, even if it's legitimate mail. To protect against spam and abuse, mail servers temporarily reject messages until the sending rate subsides. This is also closely related to too many concurrent connections errors.
Furthermore, issues with DNS resolution can trigger this error. If the recipient's mail server has trouble performing DNS lookups for your sending domain (e.g., your SPF record or DKIM record), it might temporarily reject the message. This often manifests as a temporary lookup failure. Greylisting, a common anti-spam technique, also causes temporary rejections for emails from unrecognized senders, expecting a retry later to verify legitimacy. Additionally, the content of your email could trigger a high spam score on the recipient's end, leading to a temporary block or blacklisting (or blocklisting) by their anti-spam filter, particularly if there are issues with the email's formatting or suspicious links.
Pinpointing the precise problem
To effectively resolve a 451 4.3.0 error, you must pinpoint its exact cause. Start by examining the bounce message itself. Sometimes, the message might contain additional details, such as a specific sub-code beyond 4.3.0, or a more descriptive text indicating the problem, such as temporary lookup failure or resource limit exceeded. This supplementary information is crucial for diagnosis.
Next, review your mail server logs. These logs provide detailed records of email transactions, including the precise SMTP responses received from destination servers. Look for entries corresponding to the bounced emails, noting the exact error message and any preceding or succeeding lines that might offer context about the connection, authentication, or content scanning. Here is an example of what you might see:
Example Postfix log entry for 451 4.3.0 errorlog
Jun 15 10:30:45 mailserver postfix/smtp[12345]: CD789AB: to=<recipient@example.com>, relay=mail.destination.com[192.0.2.1]:25, delay=0.5, delays=0.1/0.1/0.2/0.1, dsn=4.3.0, status=deferred (host mail.destination.com[192.0.2.1] said: 451 4.3.0 Mail server temporarily rejected message (in reply to end of DATA command))
If you manage the recipient's mail server, checking its logs is even more insightful. The logs on the receiving end will often provide a more granular reason for the temporary rejection, such as specific internal errors, anti-spam verdicts, or resource alerts. Accessing these logs is paramount for server administrators to address underlying infrastructure issues or policy configurations.
Common causes
Symptoms/log indicators
Server resource exhaustion
Logs show disk full, out of memory, or high CPU usage warnings. Messages backlog in queue.
Rate limiting/throttling
Error messages like too many connections, rate exceeded. Occurs when sending high volume.
DNS issues
Log entries such as temporary lookup failure or unresolvable domain.
Greylisting/Spam filter
Initial rejections followed by successful delivery on retry. Logs may mention greylisted or spam detected.
Effective strategies for resolution and prevention
Resolving the 451 4.3.0 error depends on its root cause and whether you control the sending or receiving server. Since it's a temporary error, patience and systematic troubleshooting are key.
For email senders
Allow retries: Most MTAs automatically retry messages after a 451 error. If delivery eventually succeeds, the problem might have been a transient server glitch or a greylisting event.
Monitor your sending volume: If you're sending high volumes, try to reduce your sending rate or spread it out to avoid hitting recipient server limits. This can prevent rate limiting issues.
Verify DNS records: Ensure your domain's SPF, DKIM, and DMARC records are correctly configured and propagated. DNS lookup failures often trigger 451 errors, especially for domains with poor configuration. For example, Yahoo has specific requirements regarding this.
Improve sender reputation: A good sender reputation can help you avoid temporary rejections related to spam filters. Focus on sending desired content to engaged recipients. If you've been on a blacklist (or blocklist), it can take time to recover.
For recipient server administrators
Check server resources: Monitor CPU, memory, and disk usage. Address any bottlenecks or insufficient resources. This might involve upgrading hardware, optimizing server configurations, or clearing disk space.
Review mail server configuration: Ensure your SMTP server is configured correctly. Incorrect settings in Postfix, Exchange, or other mail servers can lead to internal processing errors.
Adjust anti-spam/greylisting policies: If legitimate emails are consistently deferred due to greylisting or spam filters, consider fine-tuning your policies or whitelisting known trusted senders. For instance, sometimes a 451 4.3.2 error in China points to such policies.
Best practices for prevention
Proactive measures are always better than reactive troubleshooting. By maintaining a healthy email ecosystem, you can significantly reduce the occurrence of 451 4.3.0 errors.
Regular monitoring: Implement robust monitoring for your mail servers' resources and logs. For senders, keep an eye on your domain reputation.
Proper DNS setup: Maintain accurate and complete SPF, DKIM, and DMARC records to ensure proper authentication and reduce the likelihood of temporary rejections by recipient servers. This is fundamental for good email deliverability.
List hygiene: Regularly clean your email lists to remove invalid or inactive addresses. Sending to clean lists reduces bounce rates and improves sender reputation.
Content quality: Avoid common spam trigger words and maintain a healthy text-to-image ratio. Emails that look like spam are more likely to be temporarily rejected or blocklisted.
Views from the trenches
Best practices
Always include relevant DNS records: SPF, DKIM, and DMARC are crucial for validating your sending domain and helping recipient servers trust your mail. Misconfigurations can easily lead to temporary rejections.
Maintain consistent sending volumes: Avoid sudden spikes in email volume from a new IP or domain, as this can trigger rate limits and temporary blocks by recipient servers.
Regularly monitor server resources: Ensure your mail server (or your ESP’s servers) has sufficient disk space, CPU, and memory to handle your email traffic without becoming overloaded.
Keep an eye on your sender reputation: Proactively monitor for blacklistings (or blocklistings) and spam complaints, as poor reputation can increase the likelihood of temporary rejections.
Common pitfalls
Ignoring additional error details: Often, the bounce message or server logs provide more specific information beyond just '451 4.3.0', such as 'temporary lookup failure' or 'queue file write error'.
Not checking recipient server policies: Some recipient domains use aggressive anti-spam policies or greylisting, which will always result in an initial 451, expecting a retry.
Overlooking network issues: Temporary network instability between your sending server and the recipient's server can cause connection timeouts and lead to 451 errors, which are not always evident.
Sending to unengaged lists: High bounce rates from stale or invalid addresses can signal poor list hygiene and contribute to a negative sender reputation, leading to temporary rejections.
Expert tips
Utilize DMARC reports to identify 451 patterns: Aggregate reports can show which receivers are deferring your mail and highlight potential configuration or volume issues.
Perform targeted resends for 4xx errors: If a specific message batch receives many 451s, consider resending smaller segments over a longer period.
Check for blacklisting/blocklisting on major lists: Even a temporary listing on a DNSBL can cause broader temporary rejections.
Engage directly with postmasters for persistent issues: If problems persist with a specific domain, reaching out to their postmaster can help resolve unique policy blocks.
Expert view
Expert from Email Geeks says that the exact reason for a 451 4.3.0 error cannot be given without more details, as it could be anything from triggering a high spam score to an internal load or scanning issue on the recipient's server that caused a deferral.
May 16, 2023 - Email Geeks
Expert view
Expert from Email Geeks says that a 451 error typically means trying again may succeed or result in another SMTP response code with more details.
May 16, 2023 - Email Geeks
Key takeaways
The 451 4.3.0 'Mail server temporarily rejected message' error is a common but often manageable email delivery issue. Its temporary nature means that, in many cases, simply waiting and allowing your mail server to retry will resolve the problem. However, repeated occurrences signal deeper issues that warrant investigation.
By understanding the potential causes, from server overload and rate limiting to DNS misconfigurations and spam filtering, and by implementing proactive measures, you can significantly reduce the impact of these temporary rejections. Regular monitoring, proper authentication, and good sending practices are your best defense against 451 errors and essential for maintaining robust email deliverability.
