What causes Microsoft S3150 blocklist bounces and how can I resolve them?
Michael Ko
Co-founder & CEO, Suped
Published 23 May 2025
Updated 27 Aug 2025
5 min read
Getting an email bounce message with a code like S3150 from Microsoft domains can be incredibly frustrating. It usually indicates that your sending IP address, or a portion of its network, has been placed on a Microsoft blocklist. While sometimes it's a transient technical issue on Microsoft's end, it more often points to an underlying problem with your email sending practices or sender reputation. Understanding what triggers these blocks is the first step toward getting your emails delivered reliably.
The S3150 error message explicitly states, "Please contact your Internet service provider since part of their network is on our block list (S3150)." This means that Microsoft's systems have identified your sending IP (or a range of IPs) as suspicious or associated with poor sending behavior, leading to a block or throttling of your email traffic. While S3150 typically implies throttling, meaning emails are still delivered but at a reduced rate, it can quickly escalate to a full block if the issues persist.
When you encounter this, it's essential to investigate rather than assume it's just a temporary glitch. While there have been instances where Microsoft acknowledges technical difficulties on their side, consistently seeing S3150 indicates that you need to take proactive steps to diagnose and resolve the root causes affecting your email deliverability to their mailboxes. Addressing these issues can significantly improve your sender reputation and ensure your messages reach their intended recipients.
Microsoft's email infrastructure is designed to protect its users from unwanted mail. When their systems detect patterns indicative of spam or malicious activity, they take measures to block (or blocklist) the source. Several factors contribute to an IP address being flagged with an S3150 error.
A common cause is a poor sending reputation, often stemming from high spam complaint rates, sending to invalid or inactive email addresses, or a sudden increase in email volume from a new or unestablished IP. Even if you're not intentionally sending spam, poor list hygiene or a compromised sending system can lead to these issues.
Misconfigured DNS records such as missing or incorrect SPF, DKIM, and DMARC records, can also trigger blocks. Microsoft and other major ISPs increasingly rely on these authentication protocols to verify sender legitimacy. A lack of proper authentication or alignment can lead to emails being marked as suspicious, contributing to a diminished sender reputation and potential blocklisting.
Understanding the S3150 error
The S3150 error from Microsoft indicates that your IP address has been placed on a blocklist or is being throttled due to suspicious activity. This differs from other common bounce codes like S3140, which signifies a full block. While throttling allows some messages through, it's a clear warning that action is needed to prevent complete blocking. You can learn more about how to resolve Microsoft Outlook S3140 errors if you encounter that specific issue.
Steps to resolve a Microsoft S3150 block
When facing an S3150 bounce, the immediate goal is to get your IP delisted (or un-throttled) and restore normal email flow. This often involves a two-pronged approach: addressing the underlying issues and submitting a delisting request to Microsoft's Sender Information for Outreach team.
The delisting process typically requires you to provide details about the blocked IP, the error message, and the steps you've taken to resolve the issue. Be prepared to explain how you've improved your sending practices or corrected any DNS configuration problems. Being thorough and transparent can expedite the review process. For detailed troubleshooting, refer to our guide on how to resolve email blocking issues with Microsoft domains.
Immediate actions
Identify the sending IP: Pinpoint the exact IP address generating the S3150 bounce.
Prevention is always better than cure in email deliverability. To avoid future S3150 blocklist bounces, focus on maintaining a stellar sender reputation and adhering to email sending best practices. This includes warming up new IPs, sending relevant content, and segmenting your lists effectively.
Regularly monitoring your email metrics, such as open rates, click-through rates, and complaint rates, can provide early warnings of potential issues. High bounce rates or a sudden spike in complaints are red flags that should prompt immediate investigation. You can conduct a free online email deliverability test to diagnose any problems.
Beyond technical configurations, user engagement plays a crucial role. If recipients aren't interacting with your emails, it can signal to ISPs that your content is irrelevant or unwanted. Encourage engagement through clear calls to action and personalized content. Also, ensure a clear and easy unsubscribe option is available in every email.
Views from the trenches
Best practices
Actively monitor all bounce messages, especially for specific error codes like S3150 and S3140.
Regularly check your sending IPs against public and private blocklists to catch issues early.
Maintain strong sender authentication (SPF, DKIM, DMARC) for all sending domains.
Implement a consistent email list cleaning strategy to remove inactive or invalid addresses.
Common pitfalls
Ignoring S3150 as a minor issue, allowing it to escalate to full blocks.
Failing to submit a delisting request to Microsoft after resolving underlying problems.
Sending emails to purchased or old lists, leading to high bounce and complaint rates.
Not having proper SPF, DKIM, and DMARC records, which signals untrustworthiness to ISPs.
Expert tips
Keep detailed records of all bounce messages and communication with Microsoft support.
If Microsoft initially denies an issue, re-submit with more detailed bounce logs.
Periodically review your email sending infrastructure for any vulnerabilities or compromises.
Educate your marketing team on deliverability best practices to avoid common pitfalls.
Marketer view
Marketer from Email Geeks says they started experiencing a high volume of S3150 bounces about four days prior to the thread, suggesting a broader issue.
2025-08-18 - Email Geeks
Marketer view
Marketer from Email Geeks says that after contacting support, their IP blocks were instantly lifted, indicating that sometimes raising a ticket is necessary.
2025-08-18 - Email Geeks
Maintaining healthy email deliverability with Microsoft
Microsoft S3150 blocklist bounces are a clear indicator that your email sending practices need attention. While occasional technical glitches on Microsoft's side can occur, persistent S3150 errors typically point to issues with your sender reputation, email authentication, or list hygiene. By understanding the causes and implementing a proactive strategy for monitoring and maintaining your email deliverability, you can significantly reduce the likelihood of encountering these blocks.
Remember, email deliverability is an ongoing effort. Regular review of your sending metrics, prompt action on bounce messages, and adherence to industry best practices are key to ensuring your emails consistently reach the inbox, especially with major providers like Microsoft.
Microsoft domains can be incredibly frustrating. It usually indicates that your sending IP address, or a portion of its network, has been placed on a Microsoft blocklist. While sometimes it's a transient technical issue on Microsoft's end, it more often points to an underlying problem with your email sending practices or sender reputation. Understanding what triggers these blocks is the first step toward getting your emails delivered reliably.
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Microsoft indicates that your IP address has been placed on a blocklist or is being throttled due to suspicious activity. This differs from other common bounce codes like S3140, which signifies a full block. While throttling allows some messages through, it's a clear warning that action is needed to prevent complete blocking. You can learn more about how to resolve Microsoft Outlook S3140 errors if you encounter that specific issue.
