Why are IPs listed as blocked on SNDS with no Microsoft bounces?
Matthew Whittaker
Co-founder & CTO, Suped
Published 30 Apr 2025
Updated 19 Aug 2025
10 min read
It's a common head-scratcher for email senders: you log into Microsoft Smart Network Data Services (SNDS) and see your IP address listed as blocked (or blacklisted, we'll use both terms), yet you're not seeing a surge of bounce messages from Microsoft domains. This can be incredibly confusing and often leads to questions about what's truly happening with your email deliverability to Outlook, Hotmail, and other Microsoft properties. You might be asking yourself, "If I'm blocked, why am I not bouncing?"
The discrepancy between what SNDS reports and what your bounce logs show isn't as uncommon as you might think. Microsoft's email ecosystem is vast and complex, with multiple layers of filtering and reputation systems working in tandem. SNDS provides valuable insights, but it doesn't always tell the complete story of how your mail is being handled. This guide aims to shed light on why you might observe this seemingly contradictory behavior and what it means for your sending reputation and inbox placement.
Understanding these nuances is crucial for maintaining good sender reputation and ensuring your emails reach their intended recipients, not the junk folder or, worse, nowhere at all. We will explore the various reasons behind this phenomenon and offer actionable steps to troubleshoot and improve your email deliverability, even when SNDS seems to be sending mixed signals.
Understanding Microsoft's filtering layers
Microsoft's email infrastructure employs a multi-faceted approach to filtering incoming mail. While SNDS reports offer a public view into an IP's reputation and potential blocklisting, they do not encompass every internal decision made by Microsoft's SmartScreen filters. An IP address listed as "blocked" on SNDS might indicate a broad issue, but it doesn't necessarily translate to immediate, hard bounce messages for every email you send.
One key aspect to consider is the difference between a hard block (resulting in a bounce) and a softer form of filtering. Microsoft (like other major mailbox providers) often employs "greylisting" or "silent dropping" of emails. In these scenarios, mail might be deferred, delayed, or even accepted for delivery but then routed directly to the spam or junk folder without generating a bounce notification back to the sender. This can make it challenging to diagnose the true impact of a blocklist entry reported in SNDS, as your sending logs might show successful deliveries, even if the emails are not reaching the inbox.
Furthermore, Microsoft's internal blocklists (or blacklists) operate dynamically. An IP could be flagged for various reasons, from sustained low engagement to a sudden spike in spam complaints, which might trigger an initial blocklisting in SNDS. However, the system might not immediately issue bounce codes if it detects a potential for improvement or if the block is intended to be a temporary measure. This allows the system to throttle or degrade delivery without outright rejecting all mail, giving senders a chance to correct issues before a more severe block is enforced. For more insights into these dynamics, consider how to resolve Microsoft email blocks even when SNDS shows a normal status.
Why your IP might be blocklisted without bounces
There are several underlying factors that can contribute to an IP being listed as blocked on SNDS without corresponding bounce messages:
Spam complaints: Even a low volume of spam complaints can significantly impact your sending reputation, prompting Microsoft to block (or blacklist) your IP. These complaints don't always result in hard bounces, especially if the volume is below a critical threshold that would trigger an immediate rejection. Instead, mail might be silently junked. This is a common reason why transactional deliverability issues with Microsoft persist despite a green SNDS status.
Reputation lag: SNDS data can sometimes lag behind real-time reputation changes. An IP might have been blocklisted in the past, but the issue has since been resolved, yet the SNDS status hasn't fully updated. Conversely, a new issue might be developing, but the full impact (like bounces) hasn't materialized yet. This lag can also explain why SNDS data might contradict email deliverability during IP warming.
Internal filters: Microsoft maintains numerous internal, dynamic filters that assess content, sender behavior, and IP reputation. An IP might be in a state where these internal filters are directing mail to the junk folder or deferring it, rather than issuing a bounce, especially if the system is trying to avoid giving spammers explicit feedback.
Domain reputation: While SNDS focuses on IP reputation, your sending domain's reputation is equally, if not more, critical. A poor domain reputation can lead to filtering even if your IP is not explicitly hard-blocked. Mailbox providers assess multiple factors, and if your domain is associated with spammy practices, messages may be filtered to junk, regardless of the IP status in SNDS. Learn more about understanding your email domain reputation.
This multifaceted filtering means that a "blocked" status on SNDS is a strong indicator of a problem, even if it doesn't manifest as immediate bounce messages. It signals that your mail is facing significant deliverability challenges within the Microsoft ecosystem.
Troubleshooting when SNDS reports a block
When you observe this discrepancy, proactive measures are key to diagnosing and resolving the underlying issues. Simply waiting for bounces that may never come won't improve your inbox placement. The first step is to recognize that a "blocked" status in SNDS is a red flag, regardless of bounce messages.
Start by thoroughly examining your email logs for any signs of deferrals, delays, or unusual connection issues when sending to Microsoft domains. Sometimes, emails are not hard rejected, but rather soft bounced with a transient error that indicates a temporary issue, which can still signify a reputation problem. You should also broaden your monitoring beyond bounce logs. Use an email deliverability testing tool to verify inbox placement across various email clients, including those powered by Microsoft. If your emails are consistently landing in the spam folder, that's a clear sign of an underlying reputation issue, even without bounce notifications.
Engaging directly with Microsoft is also a viable step. While their support responses can sometimes be generic, providing them with specific IP addresses, sending timestamps, and sample email headers can help them investigate. You can use the Microsoft 365 anti-spam IP Delist Portal to attempt delisting your IP, even if you're not seeing bounces. Remember, a block (or blacklist entry) on SNDS indicates a problem with your IP's reputation in their system. For more information, refer to the SNDS FAQ for official Microsoft guidance.
Furthermore, consider the broader context of your email program. Are you practicing good list hygiene? Are you sending to engaged recipients? Have there been any recent changes in your sending volume, content, or infrastructure? These factors can all influence your standing with Microsoft, even if hard bounces aren't occurring. Proactive monitoring and adherence to best practices are crucial for sustainable email deliverability.
Long-term strategies for reputation management
Beyond immediate troubleshooting, a long-term strategy for maintaining excellent sender reputation is essential. This involves consistent application of email authentication protocols and adherence to best practices.
Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. Microsoft heavily relies on these protocols for authenticating legitimate senders. Proper alignment helps build trust and signal to mailbox providers that your emails are not spoofed or malicious. If your emails fail at Microsoft due to SPF issues, it can indirectly affect your IP's reputation in SNDS.
Maintaining a clean and engaged subscriber list is also paramount. Regularly remove inactive or unengaged subscribers to reduce the likelihood of hitting spam traps or generating complaints. High engagement rates and low complaint rates are strong positive signals that can counteract a negative SNDS blocklist (or blacklist) status. Consider segmenting your audiences and tailoring your content to improve engagement. This proactive approach helps your email deliverability into the inbox for 2025 and beyond.
Finally, be patient and consistent. Reputation building is a marathon, not a sprint. Even after resolving issues, it can take time for SNDS to reflect positive changes, and for Microsoft's internal systems to fully trust your sending practices again. Continued adherence to best practices will eventually lead to improved deliverability. To understand more about the inner workings of email blocklists, dive into our comprehensive guide.
Views from the trenches
Best practices
Maintain consistent sending volumes and patterns, avoiding sudden spikes that might look suspicious.
Regularly monitor your engagement metrics, including opens, clicks, and complaint rates.
Implement a double opt-in process for all new subscribers to ensure list quality.
Use clear and prominent unsubscribe options to minimize spam complaints.
Common pitfalls
Relying solely on SNDS status without checking other deliverability metrics like inbox placement.
Ignoring low complaint rates or silent filtering, assuming no bounces means no problem.
Sending to very old or unengaged email lists, which can trigger spam traps.
Not engaging with Microsoft support or using their delisting tools when issues arise.
Expert tips
A "blocked" status on SNDS doesn't always mean your emails are bouncing; they might be going to junk.
Microsoft's internal filtering is dynamic and can override public SNDS statuses.
Domain reputation plays a significant role, sometimes more than IP reputation alone.
Proactive list hygiene and strong authentication are key to long-term deliverability.
Marketer view
Marketer from Email Geeks says they have seen IPs listed on SNDS as "blocked" without corresponding Microsoft bounces, usually due to people marking emails as spam.
2020-04-03 - Email Geeks
Marketer view
Marketer from Email Geeks says they have seen false information in SNDS, where a customer with a 20% open rate with Microsoft domains remains listed as "blocked" for over a year due to old, resolved problems.
2020-04-03 - Email Geeks
Key takeaways
Seeing your IPs listed as blocked on SNDS without corresponding bounce messages from Microsoft can be a perplexing situation. It highlights the intricate and often non-transparent nature of email deliverability, especially with major mailbox providers like Microsoft. The key takeaway is that an SNDS block is always a signal to investigate, even if your bounce rates don't immediately reflect it.
The absence of bounces doesn't mean your emails are reaching the inbox. More often, it indicates that messages are being silently junked, deferred, or filtered to the spam folder, a more insidious problem than outright rejection. Understanding that Microsoft's internal reputation systems are more comprehensive and dynamic than what SNDS publicly displays is crucial for effective deliverability management.
By actively monitoring your inbox placement, diligently maintaining your sender reputation through authentication and list hygiene, and being prepared to engage with Microsoft support, you can mitigate these hidden deliverability challenges. Remember, the goal is not just to avoid bounces, but to ensure your emails consistently reach the primary inbox.
Proactive email deliverability management is an ongoing process that requires vigilance and adaptability. By understanding the complexities of systems like SNDS and Microsoft's filtering mechanisms, you can ensure your email program remains healthy and effective.
Microsoft. The key takeaway is that an SNDS block is always a signal to investigate, even if your bounce rates don't immediately reflect it.
