How to resolve Microsoft 451 4.7.650 error during email IP warm-up?
Michael Ko
Co-founder & CEO, Suped
Published 11 Jun 2025
Updated 19 Aug 2025
8 min read
Encountering the Microsoft 451 4.7.650 error during email IP warm-up can be incredibly frustrating. It signals that your mail server has been temporarily rate-limited due to concerns about its IP reputation. While many email senders understand the importance of a gradual IP warm-up, seeing this error often leaves them wondering if their efforts are misguided or if something more fundamental is amiss.
This deferral message indicates that Microsoft is cautious about the volume or nature of the mail originating from your IP address. It's their way of protecting their users from potential spam or unwanted mail. For senders undergoing IP warm-up, this often feels contradictory, as the goal is to build trust by sending legitimate email.
The good news is that a 451 error is a temporary deferral, not a hard block. This means your email server will retry sending the messages. However, frequent deferrals can significantly delay your email delivery and hinder your warm-up progress. It’s crucial to address the root causes to ensure smooth deliverability and build a strong sender reputation.
Understanding the Microsoft 451 4.7.650 error
The 451 4.7.650 error specifically indicates that Microsoft's systems have detected something about your IP's sending behavior that warrants temporary rate limiting. This is almost always tied to IP reputation. During IP warm-up, the goal is to gradually introduce your new IP to ISPs, building a history of positive sending behavior. However, sometimes the volume, recipient engagement, or even the content can trigger these temporary blocks.
For specific details on how Microsoft assesses IP reputation and email delivery, their official postmaster website is an invaluable resource. This error implies that while your emails aren't being permanently rejected, Microsoft is signaling caution and a need for adjustment in your sending strategy.
One common misconception is that IP warming solely involves sending high volumes. In reality, it's about establishing trust. If the volume is too low, Microsoft might not have enough data to form a positive opinion of your IP's reputation, leading to rate limiting as a precautionary measure. It's a delicate balance.
Temporary throttling: The 451 error means Microsoft is temporarily deferring your emails, indicating an IP reputation issue, often stemming from too rapid a volume increase or poor recipient engagement.
Warm-up stage: This error frequently appears during IP warm-up, as Microsoft is still evaluating your new IP's sending patterns and trustworthiness.
Common causes for rate limiting during warm-up
One of the most paradoxical reasons for receiving a 451 4.7.650 error during IP warm-up is actually not sending enough volume. Microsoft, like other major mailbox providers, needs a consistent stream of good mail to build trust in a new IP. If the volume is too sporadic or low, they may not accumulate sufficient positive signals, leading to prolonged scrutiny and rate limiting.
Another critical factor is the quality of your recipient list. Even during warm-up, sending to low-engagement users, inactive addresses, or, worse, spam traps can quickly tank your IP reputation. Microsoft heavily weighs user engagement and negative feedback (spam complaints, blocks) in their filtering decisions. For more details, you can consult this discussion on Microsoft Q&A.
While less common for a 451 error specifically, fundamental DNS issues such as missing or misconfigured PTR records, or problems with SPF, DKIM, or DMARC can also contribute to a poor sender reputation. Microsoft is particularly strict about PTR records, and a missing or incorrect one can significantly impact your standing, as can hidden SPF DNS timeouts.
Example: Checking a PTR recordBASH
dig -x 103.103.198.52
Strategies to resolve and prevent the error
Resolving the 451 4.7.650 error requires a multi-faceted approach during IP warm-up. First, it’s vital to ensure you are gradually increasing your email volume, but also to understand that backing off when throttled is just as important. Instead of pushing through, reduce your volume for a day or two and then resume the gradual increase. This tells Microsoft you are responsive to their signals.
Focusing on high-engagement recipients during warm-up is paramount. These are contacts who have recently opened, clicked, or replied to your emails. Their positive interactions help build a good reputation for your IP. Avoid sending to dormant or unengaged segments until your IP reputation is firmly established. This strategy is also crucial when you need to manage large sends without sufficient IP warm-up.
Beyond list segmentation, proactively maintaining a clean list by removing bounces and unsubscribes is crucial. Even a small number of spam complaints can significantly harm your IP's standing, especially during the sensitive warm-up phase. If Microsoft blocks your IP address during warm-up, addressing these list hygiene issues is a primary step to resolve deliverability issues.
Long-term reputation building and monitoring
Beyond immediate troubleshooting, building a robust long-term reputation with Microsoft requires consistent, positive sending behavior. This involves continuously sending relevant, wanted emails to engaged subscribers. Maintaining consistent volume, especially after warm-up, helps Microsoft understand your normal sending patterns and build a positive historical record for your IP.
Leverage Microsoft's Postmaster Tools to monitor your IP and domain reputation, spam complaint rates, and delivery errors. This data provides invaluable insights into how Microsoft views your sending. If you continue to face persistent IP reputation issues with Microsoft despite warm-ups, these tools are your first line of defense, allowing you to diagnose and resolve problems.
Another crucial aspect of long-term deliverability is actively monitoring major email blocklists (or blacklists). While the 451 4.7.650 error isn't a blocklist entry itself, poor sending practices that lead to this error can eventually land your IP on one. Regularly checking your IP against these lists helps you react quickly if you find your IPgets blocklisted during IP warmup. Maintaining strong email authentication protocols like SPF, DKIM, and DMARC is also fundamental to demonstrating legitimacy.
Key factors for improving IP reputation with Microsoft
Consistent volume: Maintain a steady sending volume, even after warm-up, to provide Microsoft with continuous positive signals.
High engagement: Prioritize sending to your most engaged subscribers to generate positive interactions and build trust.
Monitor feedback loops: Sign up for Microsoft's Junk Mail Reporting Program (JMRPP) to quickly identify and remove users who mark your emails as spam.
Ensuring smooth deliverability to Microsoft
The Microsoft 451 4.7.650 error is a clear signal that your IP warm-up process needs adjustment. It’s not just about hitting volume targets, but about building a trustworthy sending history based on positive recipient engagement and adherence to best practices. By understanding Microsoft's reputation criteria and proactively addressing any issues, you can navigate the warm-up phase successfully and ensure your emails reach the inbox reliably.
Remember, email deliverability is an ongoing process of monitoring and refinement. Even after your IPs are fully warmed, continuous vigilance over your sending practices will safeguard your sender reputation and ensure consistent inbox placement, especially with a vigilant provider like Microsoft.
Views from the trenches
Best practices
Maintain meticulous list hygiene to minimize bounces and spam complaints.
Segment your audience and prioritize sending to your most engaged subscribers first.
Slowly increase email volume during warm-up, being prepared to reduce if throttled.
Ensure all DNS records, especially PTR, SPF, and DKIM, are correctly configured.
Proactively register and monitor your IP in Microsoft Postmaster Tools.
Sending to unengaged or old email addresses, leading to high complaint rates.
Ignoring temporary deferral messages instead of adjusting sending patterns.
Not configuring reverse DNS (PTR) records correctly for your sending IP.
Failing to monitor feedback loops or Postmaster Tools for reputation insights.
Expert tips
Implement a dedicated IP warming schedule that accounts for Microsoft's unique sensitivity.
Focus on content quality and personalization to drive positive engagement metrics.
Consider segmenting warm-up by domain, giving Microsoft domains extra attention.
Engage in conversations about email deliverability to stay ahead of ISP changes.
Regularly audit your email sending infrastructure for any misconfigurations.
Marketer view
Marketer from Email Geeks says that sending only 500 emails per day is hardly any, and the sender should continue increasing volume, pulling back only if throttling occurs.
2025-03-15 - Email Geeks
Marketer view
Marketer from Email Geeks says that rate limiting could be due to insufficient volume, meaning Microsoft needs more mail to build trust, or it could be due to bad mail, which would make continuing to ramp up detrimental.
is cautious about the volume or nature of the mail originating from your IP address. It's their way of protecting their users from potential spam or unwanted mail. For senders undergoing IP warm-up, this often feels contradictory, as the goal is to build trust by sending legitimate email.
