Why are emails sent via Microsoft SMTP going to spam despite good senderscore at other ISPs?
Michael Ko
Co-founder & CEO, Suped
Published 15 May 2025
Updated 16 Aug 2025
8 min read
Many of us have encountered the puzzling scenario where emails sent via Microsoft SMTP servers are consistently landing in spam or junk folders for recipients using Outlook, Hotmail, or Live.com, even when external reputation metrics like SenderScore indicate good standing with other Internet Service Providers (ISPs). This can be particularly frustrating because it suggests a specific challenge with Microsoft's filtering mechanisms, rather than a general email deliverability problem.
The discrepancy often leads to questions about whether the issue is isolated to your sending practices or if it points to a broader, systemic problem with Microsoft's email infrastructure, such as their shared IP pools or recent changes to their spam filters. While a good SenderScore is a positive indicator, it doesn't always reflect the entire picture, especially when dealing with the nuanced and often proprietary filtering algorithms of major mailbox providers.
It's a common misconception that a single good reputation score guarantees inbox placement across all providers. Each ISP has its own set of rules and internal scoring systems that might weigh different factors more heavily. For Microsoft, this means looking beyond generic scores and delving into their specific feedback loops and internal reputation signals. Emails sent through their own SMTP are subject to this scrutiny.
Understanding Microsoft's unique filtering
Unlike some other ISPs, Microsoft employs a sophisticated filtering system that assigns a Spam Confidence Level (SCL) and Bulk Complaint Level (BCL) to incoming emails. These internal scores are crucial in determining whether an email lands in the inbox or the junk folder. A good SenderScore from a third-party doesn't necessarily translate to low SCL or BCL scores within Microsoft's ecosystem.
Microsoft's filters also prioritize user engagement and complaint rates very highly. If users consistently mark your emails as junk, even if the volume is low, Microsoft will penalize your sending reputation, leading to more emails being flagged as spam. This can happen even if your overall domain and IP reputation appear strong on other platforms. Understanding this nuanced approach is key to improving deliverability to Microsoft recipients. For more information on why emails might be hitting the junk folder, see Why are my emails going to the Outlook/Hotmail junk folder.
External reputation factors
SenderScore and similar public metrics aggregate data from various ISPs and spam trap networks. They provide a general health check of your sending infrastructure and practices across the broader internet. While useful, they might not capture the specific, proprietary filtering rules of individual major mailbox providers like Microsoft.
Broad view: Reflects general sending practices across the internet.
Data sources: Based on data from participating ISPs and public blocklists.
Microsoft's internal reputation
Microsoft's filtering prioritizes signals specific to their ecosystem, including user complaints (Junk Email Reporting Program), engagement metrics (opens, clicks), and authentication failures that they detect. They also have their own internal blocklists (blacklists) that are not publicly accessible or reflected in a SenderScore.
Specific view: Focuses on how users within Microsoft services interact with your emails.
Key metrics: Complaint rates, engagement, SCL/BCL values from Microsoft's systems.
Common causes for Microsoft SMTP deliverability issues
Several factors can contribute to Microsoft SMTP emails landing in spam, even with an otherwise good reputation. One significant factor is the shared IP addresses used by Microsoft 365. If other users on the same IP address are sending problematic content, it can negatively affect the reputation of that shared IP, impacting your deliverability. This is often an issue beyond your direct control and requires intervention from Microsoft themselves.
Content is another critical element. Even legitimate emails can be flagged if they contain spam-like language, excessive links, or poor formatting. Microsoft's filters are highly sensitive to these cues. Moreover, a sudden spike in email volume from a previously low-volume sender can also trigger spam filters, regardless of content quality. For a more general overview, read about Why your emails are going to spam in 2024.
Even with correct SPF, DKIM, and DMARC records in place, subtle configuration issues or a lack of alignment can cause problems. Microsoft is particularly strict about these authentication protocols. Ensure that your DMARC policy is set appropriately, and monitor your DMARC reports to identify any authentication failures specific to Microsoft domains. You can read more about A simple guide to DMARC, SPF, and DKIM.
Diagnosing the problem
When your emails consistently land in the spam folder at Microsoft, the first step is to perform a thorough diagnosis. Start by examining the full email headers of a message that ended up in spam. Look for the X-Forefront-Antispam-Report header, which contains the SCL (Spam Confidence Level) and BCL (Bulk Complaint Level) values. These scores provide direct insight into how Microsoft is classifying your emails.
A high SCL score (e.g., 5-9) indicates that Microsoft believes the email is spam, while a high BCL score (e.g., 5-9) means it's perceived as bulk mail with a high complaint rate. Both can lead to the junk folder. Compare these values with emails that successfully reach the inbox from other providers to pinpoint the specific Microsoft-centric issue. You can learn more about sender reputation generally here.
SCL and BCL values
Even if your SenderScore looks good, high SCL (Spam Confidence Level) or BCL (Bulk Complaint Level) scores from Microsoft's filters are a strong indicator of problems. These internal scores are proprietary to Microsoft and determine junk folder placement. A score of 5 or higher for SCL often means junk folder, and high BCL suggests users are complaining about your mail.
Additionally, check your blocklist (or blacklist) status. While you might not be on major public blocklists, Microsoft maintains its own internal blocklists (blacklists). If your IP address or domain is listed on one of these, it will significantly impact deliverability. You may need to engage directly with Microsoft's support channels to investigate and request delisting, especially if you suspect issues with their shared IP pools affecting your sends.
Strategies to improve deliverability
To improve deliverability when sending emails via Microsoft SMTP, focus on strategies that align with Microsoft's filtering priorities. Ensure all your email authentication protocols, including SPF, DKIM, and DMARC, are correctly configured and pass validation. Pay close attention to DMARC alignment, as this is a common point of failure for emails being sent through third-party or shared SMTP services, even if the primary authentication checks pass. For specific fixes, consider how to fix common DMARC issues in Microsoft 365.
Beyond technical configurations, focus on content and engagement. Craft clear, concise email content free of spam trigger words or overly promotional language. Ensure your emails provide clear value to the recipient and encourage interaction. Regularly clean your email list to remove inactive or invalid addresses, as high bounce rates and low engagement can severely damage your sender reputation with Microsoft. Maintaining good list hygiene also reduces the risk of hitting spam traps, which are a major red flag for any ISP. For more about the death of IP-based reputation, this resource offers valuable insights into the evolving landscape of deliverability.
Factor
Impact on Microsoft deliverability
Action to take
Email authentication (SPF, DKIM, DMARC)
Crucial for establishing sender legitimacy; Microsoft is strict on alignment.
Verify all records are correct and ensure DMARC alignment passes for emails via Microsoft.
Content quality
Spammy language, poor formatting, or excessive links can trigger filters.
Optimize content, use clear calls to action, and avoid spam trigger words.
User engagement
Low open/click rates and high complaint rates significantly harm reputation.
Encourage interaction, implement a clear unsubscribe option, and monitor feedback loops.
List hygiene
High bounce rates, especially to invalid addresses or spam traps, reduce trust.
Regularly clean your email list and remove unengaged subscribers.
Lastly, consider the type of emails you're sending. Transactional emails (like order confirmations or password resets) generally have higher deliverability rates due to expected engagement. Marketing emails, especially those sent to a broad audience, require more stringent practices. If you're sending marketing communications, investing in a dedicated Email Service Provider (ESP) separate from your corporate Microsoft SMTP might be beneficial to isolate your reputation.
What to do next
Navigating Microsoft's email filtering can be challenging, especially when external SenderScore metrics suggest good standing. The key lies in understanding that Microsoft employs its own intricate internal reputation system, heavily influenced by user engagement, complaint rates, and the quality of their shared IP pools. While a good external reputation is foundational, it's not the sole determinant for inbox placement with Microsoft mail services.
The path to improved deliverability involves a multi-faceted approach. This includes meticulous attention to authentication standards, optimizing content for recipient engagement, maintaining rigorous list hygiene, and proactively monitoring Microsoft's specific metrics like SCL and BCL. By focusing on these areas, you can significantly enhance your chances of reaching the inbox rather than the spam folder when sending via Microsoft SMTP.
Views from the trenches
Best practices
Actively monitor your SCL and BCL scores within Microsoft email headers for direct insights into deliverability.
Implement a clear, easy-to-find unsubscribe mechanism in every email to reduce spam complaints.
Regularly clean your email lists to remove inactive users, invalid addresses, and potential spam traps.
Common pitfalls
Relying solely on external SenderScore for Microsoft deliverability; their internal metrics are more critical.
Sending high volumes of marketing emails through Microsoft 365 SMTP without a dedicated ESP.
Ignoring user complaints or low engagement rates, which significantly impact Microsoft's internal reputation.
Expert tips
If using Microsoft 365 for outbound mail, engage their support if you suspect a shared IP reputation issue.
Consider separating transactional and marketing email streams to protect your primary domain's reputation.
Pay close attention to Microsoft's postmaster guidelines and feedback loops for specific insights.
Expert view
Expert from Email Geeks says that going into spam at Microsoft is often a warning that a block is coming and usually worsens over time. If you are paying Microsoft for outbound mail service and they are failing to deliver to Microsoft domains, you should have recourse to have that investigated.
2023-03-17 - Email Geeks
Marketer view
Marketer from Email Geeks says that they frequently encounter Microsoft-based domains with poor IP reputation, even when it was previously fine, and questioned if this was related to a recent Microsoft spam filter issue.
Microsoft SMTP servers are consistently landing in spam or junk folders for recipients using Outlook, Hotmail, or Live.com, even when external reputation metrics like SenderScore indicate good standing with other Internet Service Providers (ISPs). This can be particularly frustrating because it suggests a specific challenge with Microsoft's filtering mechanisms, rather than a general email deliverability problem.
