Send & Receive Issues

How to Resolve Microsoft SMTP Error 451 4.7.650

WPS Editor Sep 29, 2026 868 views

Question details

The user needs to resolve an SMTP delivery failure where Microsoft servers reject or delay outbound emails with the specific error code 451 4.7.650.

How to Resolve SMTP Error 451 4.7.650 from Microsoft

Product: Microsoft Exchange
Device & OS: not provided
Scenario: Sending emails to Microsoft Office 365 or Exchange domains where the receiving server responds with a temporary throttling or rate limiting error.
Observed behavior: Messages fail to deliver and return a bounce message with SMTP error 451 4.7.650, even though SPF, DKIM, and DMARC authentication records are correctly configured.

Before you start

Gather the complete non-delivery report (NDR) or bounce message you received, as it contains critical diagnostic codes and IP information needed for troubleshooting. Be sure to redact any confidential data before sharing it with support teams.

Solution 1Recommended

Adjust Sending Practices and Verify IP Reputation

Resolve rate limiting by monitoring your sending volume, eliminating email bursts, and ensuring your IP maintains a healthy reputation with Microsoft.

SMTP error 451 4.7.650 is a server-side response from Microsoft indicating that your sending IP address is being temporarily throttled. This usually happens when the receiving server detects an unusual spike in email volume or suspects spam-like behavior.

Review sending volume

Check your mail server logs to identify any sudden spikes in outgoing email volume to Microsoft domains. Reduce the frequency and send messages in smaller, spread-out batches to avoid triggering rate limits.

Check IP reputation

Use online IP reputation tools or access the Microsoft Sender Support portal to verify that your mail server's IP address has not been flagged for suspicious activity or placed on a blocklist.

Analyze email content

Review the content of the blocked emails. Ensure that your transactional or bulk messages do not contain characteristics commonly classified as spam, which can negatively impact your sender reputation.

Adjust Sending Practices and Verify IP Reputation

Authentication Does Not Prevent Throttling: Correctly configuring your SPF, DKIM, and DMARC records is essential for email security, but it does not bypass Microsoft's reputation-based throttling and rate-limiting restrictions.

Solution 2

Contact Microsoft Support for Throttling Review

If legitimate email traffic continues to be throttled after adjusting sending habits, you may need to request Microsoft to evaluate your IP address.

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Frequently Asked Questions

What does SMTP Error 451 4.7.650 mean?

This error indicates that Microsoft's receiving servers are temporarily throttling or rate-limiting emails from your sending IP address. This is typically triggered by a sudden burst in email volume or a degraded IP sender reputation.

Will fixing SPF, DKIM, and DMARC resolve this error?

No. While having correctly configured SPF, DKIM, and DMARC records is crucial for standard email delivery and security, it does not guarantee exemption from Microsoft's reputation-based rate limiting systems.

How long does a temporary SMTP 451 throttling last?

Temporary throttling usually resolves automatically within 24 to 48 hours if you stop sending bursts of emails and normalize your traffic. If the issue persists beyond this timeframe, you may need to contact Microsoft Support for a manual review.

Where can I find the diagnostic information for this error?

The diagnostic information is located in the non-delivery report (NDR) or bounce message you receive in your inbox when the email fails to deliver. Always collect this complete message when troubleshooting mail flow issues.

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