How to troubleshoot Microsoft email deliverability issues and analyze SNDS data?
Michael Ko
Co-founder & CEO, Suped
Published 26 Jul 2025
Updated 18 Aug 2025
8 min read
Dealing with email deliverability issues, especially when sending to Microsoft domains like Outlook.com and Office 365, can be a complex challenge. It often feels like you're navigating a black box, with emails sometimes going missing or landing in the junk folder without clear explanations.
While tools like Smart Network Data Services (SNDS) exist to provide some insight, many senders find the data limited or inconsistent. This can be particularly frustrating when your internal monitoring shows everything passing, yet engagement metrics suggest otherwise. It's not uncommon to see deliverability fluctuating, even after careful warmup processes and authentication.
This guide will walk you through effective strategies to troubleshoot Microsoft email deliverability issues and make the most of SNDS data, while also addressing its common limitations. We'll cover everything from understanding their filtering mechanisms to advanced reputation management techniques, helping you get your messages to the inbox.
Understanding Microsoft's filtering mechanisms
Microsoft's email filtering, particularly for Outlook and Office 365, relies on a complex interplay of factors to determine inbox placement. Unlike some other providers, their systems dynamically assess various signals, assigning an internal score. This score, often visible as the Spam Confidence Level (SCL) or Bulk Complaint Level (BCL) in email headers, indicates how likely an email is to be junk. A high SCL or BCL score means your email is more likely to land in the spam folder or be rejected.
Sender reputation is paramount. Microsoft constantly evaluates your sending IP and domain based on historical performance. This includes factors such as spam complaint rates, bounce rates, and engagement. If your sender reputation declines, even if your authentication (SPF, DKIM, DMARC) is perfectly aligned, you could face deliverability challenges. It's a continuous balancing act to maintain a positive standing.
Understanding the nuances between B2B and B2C email deliverability is also crucial. In B2B environments, recipients might report emails as spam without you ever knowing, especially if they are prospects who haven't directly engaged recently. This type of silent feedback can still impact your reputation and make troubleshooting more difficult, as traditional metrics might not capture the full picture. It's a common challenge that requires a proactive approach to list management and engagement.
Leveraging SNDS for basic insights
Microsoft Smart Network Data Services (SNDS) is a free postmaster tool designed to give senders insights into their IP reputation and email activity. To access SNDS, you need to register your sending IP addresses. Once registered, the platform provides daily data on email volume, spam complaint rates, and the overall health of your IP reputation through a color-coded system: green for good, yellow for potential issues, and red for serious problems. It also shows if your IP is listed on any internal blocklists (or blacklists) used by Microsoft.
While SNDS provides some valuable high-level data, many experienced senders find its insights can be quite limited. For instance, it provides aggregated data for IP addresses, not granular details for specific domains or campaigns. This makes it difficult to pinpoint the exact cause of a deliverability issue if you're sending from a shared IP or multiple subdomains. Furthermore, SNDS primarily offers data relevant to consumer-facing Microsoft services like Outlook.com and Hotmail, but often lacks deep insights for corporate Microsoft 365 environments, especially for paid customer data.
Despite these limitations, SNDS can still be a starting point for diagnosing issues, particularly if you see a sudden drop in your IP reputation or an increase in complaints. However, it shouldn't be your sole source of truth for email deliverability to Microsoft domains. You may find that your SNDS status looks normal, even when experiencing blocks. This is a common frustration and highlights the need for a more comprehensive troubleshooting approach, incorporating other data points and testing methods.
SNDS data limitations
While SNDS offers some insights into your IP reputation with Outlook.com and other consumer Microsoft domains, it typically lacks granular data for enterprise-level Office 365 environments. This means that even if SNDS shows a green status, you might still experience deliverability issues, particularly with B2B recipients.
For inconsistent deliverability or if SNDS data seems unhelpful, look beyond the tool. Analyze email headers for SCL/BCL scores, monitor user engagement closely, and consider employing a more robust deliverability monitoring platform to get a clearer picture.
Common troubleshooting steps and authentication
The foundation of good email deliverability lies in proper authentication. Ensuring your SPF, DKIM, and DMARC records are correctly configured and aligned is non-negotiable for Microsoft. These records help mailbox providers verify that your emails are legitimate and prevent spoofing. Even a minor misconfiguration can lead to significant deliverability problems. Regularly review these settings, especially after any changes to your sending infrastructure or ESP.
DNS configuration issues are a frequent culprit behind email delivery failures. Microsoft, in particular, can be strict about DNS lookups for SPF records, sometimes leading to a hidden SPF DNS timeout. This occurs when your SPF record requires too many DNS lookups, exceeding the 10-lookup limit and causing emails to fail authentication. Additionally, ensuring your DNS records are consistent across all sending domains is vital.
Beyond technical setup, list hygiene and engagement management are critical. If your engagement rates are declining, it could signal a problem with your list quality or content relevance. Actively suppressing inactive or unengaged contacts, even those who have only opened emails in the past, can significantly improve your sender reputation and prevent your emails from hitting spam traps or being marked as junk. Proactive list management helps maintain a healthy sending environment and improves overall inbox placement.
Example email headers for deliverability analysisplain
Received-SPF: Pass (sender IP is 1.2.3.4)
Authentication-Results: spf=pass (sender IP is 1.2.3.4); ...
X-Forefront-Antispam-Report: ... SCL:5 BCL:0 ...
Reactive list cleaning
Wait for problems: Only remove bounces or spam complaints after they occur.
Focus on metrics: Solely rely on ESP metrics for list health indicators.
Risk reputation damage: Higher chance of hitting spam traps or being blocklisted (blacklisted).
Proactive engagement management
Segment disengaged users: Create segments for contacts who haven't opened or clicked in a long time.
Re-engagement campaigns: Attempt to re-engage with a specific content strategy.
Temporary suppression: Temporarily suppress disengaged contacts to protect sender reputation.
Advanced strategies and reputation management
For organizations with diverse sending needs, using subdomains can be a powerful strategy to isolate email reputation. For example, you might use one subdomain for transactional emails and another for marketing campaigns. This way, if your marketing efforts accidentally trigger spam complaints, it's less likely to impact the deliverability of critical transactional emails. However, simply using subdomains isn't always a silver bullet; sometimes, a completely different sending infrastructure might be required for problematic email streams.
Relying solely on seed list tests can sometimes provide misleading results, especially with providers like Google and Microsoft that employ highly individualized delivery decisions. Your personal inbox might receive an email in the primary tab, while a seed list test for the same campaign indicates a 100% missing rate for that provider. This discrepancy highlights that seed lists are a snapshot and not always indicative of real-world inbox placement. Supplement seed tests with actual engagement metrics and internal monitoring.
Ultimately, managing email deliverability to Microsoft domains requires continuous monitoring and adaptation. Beyond checking SNDS and authentication, it’s about understanding your audience, maintaining a clean list, and being responsive to feedback, both explicit (complaints) and implicit (engagement). If basic troubleshooting doesn't resolve deep-seated issues, considering a comprehensive deliverability audit can provide targeted solutions tailored to your specific sending patterns and challenges.
Views from the trenches
Best practices
Actively suppress disengaged contacts, even those who only opened in the past, to improve sender reputation.
Use subdomains to isolate the reputation of different email streams (e.g., transactional vs. marketing).
Regularly check email headers for SCL/BCL scores to understand Microsoft's filtering decisions.
Common pitfalls
Relying solely on SNDS data for comprehensive deliverability insights, especially for B2B.
Neglecting to remove inactive subscribers, which can lead to spam traps and lower engagement rates.
Expecting consistent results from seed list tests when mailbox providers use individual delivery decisions.
Expert tips
If your ESP is showing weird results, try sending a test email to your personal accounts to verify actual delivery.
For B2B sending, be aware that recipients can report spam without you receiving explicit feedback.
When migrating ESPs, ensure a proper warming up of new sending infrastructure, even with existing openers.
Expert view
Expert from Email Geeks says: You should email yourself and look at the BCL and SCL scores in the headers, as you can typically get a lot of information just from the email headers.
2025-04-28 - Email Geeks
Marketer view
Marketer from Email Geeks says: It is not enough to suppress bounces, unsubscribes, and spam complaints; marketers need to proactively remove or temporarily suppress disengaged contacts to maintain a high sender reputation.
2025-04-28 - Email Geeks
Final thoughts on Microsoft deliverability
Troubleshooting Microsoft email deliverability issues involves a blend of technical diligence and strategic list management. While SNDS offers some initial diagnostics, it’s rarely enough to solve complex problems, especially for B2B sending. A holistic approach that includes rigorous email authentication, proactive list hygiene, and careful monitoring of engagement signals is essential for ensuring your emails reach their intended recipients.
By understanding the nuances of Microsoft's filtering, addressing DNS pitfalls like the SPF DNS timeout, and actively managing your sender reputation, you can significantly improve your inbox placement. Remember, deliverability is an ongoing process that requires consistent effort and adaptation to maintain healthy sender-recipient relationships.
Microsoft domains like Outlook.com and Office 365, can be a complex challenge. It often feels like you're navigating a black box, with emails sometimes going missing or landing in the junk folder without clear explanations.
Outlook.com and other consumer Microsoft domains, it typically lacks granular data for enterprise-level Office 365 environments. This means that even if SNDS shows a green status, you might still experience deliverability issues, particularly with B2B recipients.
Google and Microsoft that employ highly individualized delivery decisions. Your personal inbox might receive an email in the primary tab, while a seed list test for the same campaign indicates a 100% missing rate for that provider. This discrepancy highlights that seed lists are a snapshot and not always indicative of real-world inbox placement. Supplement seed tests with actual engagement metrics and internal monitoring.
