Summary
Emails sent via Microsoft SMTP may land in spam despite a good Sender Score elsewhere due to Microsoft's complex and unique filtering practices. Key factors include Microsoft's internal reputation systems, a strong emphasis on user engagement (opens, clicks), email content, recipient mail server configurations, spam filtering algorithms, user-reported spam, and properly configured authentication (SPF, DKIM, DMARC) records. Microsoft prioritizes user feedback and may use factors beyond IP reputation. In addition, domain reputation, content reputation, IP warm-up, and avoiding blacklists are also vital.
Key findings
- Internal Reputation Systems: Microsoft uses its own internal reputation system, which may differ significantly from Sender Score, affecting deliverability.
- User Engagement is Key: Microsoft places a high priority on user engagement (opens, clicks); low engagement can lead to spam classification, regardless of Sender Score.
- Content Matters: The content of emails, including links and wording, significantly impacts deliverability within Microsoft's ecosystem.
- Authentication is Critical: Correctly configured SPF, DKIM, and DMARC records are essential for Microsoft to accept emails and avoid spam filters.
- Feedback Loops: User-reported spam and feedback loops play a crucial role in determining deliverability.
- IP reputation isn't primary: IP reputation is not the primary factor in deliverability, especially with large providers like Microsoft.
- Multiple Reputations: Domain and content reputation matter.
- Blacklists: Blacklists are another factor that is considered when determining spam.
Key considerations
- Monitor SNDS: Monitor Microsoft's Smart Network Data Services (SNDS) to understand your reputation within the Microsoft network.
- Improve Engagement: Implement confirmed opt-in, segment your lists, clean inactive subscribers, and personalize content to boost user engagement.
- Check Authentication: Ensure your SPF, DKIM, and DMARC records are correctly configured and validated.
- Monitor Feedback Loops: Actively monitor feedback loops (FBLs) and promptly address any user complaints to maintain a positive sender reputation.
- Sender Information Form: If following best practices and still experiencing deliverability issues, submit a sender information form to Microsoft for investigation.
- Review Email Content: Ensure email content adheres to best practices to avoid triggering spam filters.
- IP warm-up: New IPs should be warmed up.
- Implement BIMI: New technologies are important.
What email marketers say
9 marketer opinions
Emails sent via Microsoft SMTP may land in spam despite good Sender Score at other ISPs due to Microsoft's unique filtering practices. These include internal reputation systems, a strong focus on user engagement (opens, clicks), content analysis, and potential blacklisting. Using a new IP, or not implementing BIMI can also impact deliverability.
Key opinions
- Internal Reputation: Microsoft might use its own internal reputation system, differing from Sender Score, to determine deliverability.
- User Engagement: Microsoft prioritizes user engagement (opens, clicks) heavily. Low engagement can lead to spam classification regardless of Sender Score.
- Content Analysis: The content of your emails, including links and wording, significantly impacts deliverability within Microsoft's ecosystem.
- IP Warm-up: Microsoft is strict in spam filtering, new IPs need to be warmed up, or they are likely to get flagged as spam.
- Blacklists: Check your domain against blacklists. If you're on one, you should take action to remove yourself from it.
- New Technologies: Failure to implement technologies like BIMI can cause deliverability issues.
Key considerations
- Monitor SNDS: Monitor Microsoft's Smart Network Data Services (SNDS) to understand your reputation within the Microsoft network.
- Improve Engagement: Implement confirmed opt-in, segment your lists, clean inactive subscribers, and personalize content to boost user engagement.
- Deliverability Tools: Use tools such as GlockApps or similar to test the deliverability of your emails to help detect any issues.
- Check Blacklists: Consistently check your domain against various blacklists to proactively address reputation issues.
- Sender Reputation: Sender Reputation can be influenced by domain reputation and content reputation.
- Implement New Tech: New technologies are important to email deliverability. You should implement them.
Marketer view
Email marketer from Reddit shares that Microsoft is really strict on their spam filtering algorithms. Suggests making sure you are doing a proper warm up of the IP address associated to your emails, as Microsoft is more likely to flag a new IP as spam.
26 Aug 2022 - Reddit
Marketer view
Email marketer from Sendinblue Blog discusses Microsoft places significant weight on user engagement. If recipients rarely open or interact with your emails, Microsoft’s filters are more likely to classify messages as spam, regardless of Sender Score. Ensure you are using confirmed opt-in, segmenting your lists, and cleaning inactive subscribers to improve engagement metrics. Also make sure you're following deliverability best practices.
3 Jul 2022 - Sendinblue Blog
What the experts say
7 expert opinions
Emails sent via Microsoft SMTP may land in spam despite good Sender Score due to factors beyond IP reputation. These include internal settings, content quality, authentication (SPF, DKIM, DMARC), user engagement, and domain reputation. Sender Score may not be relevant for O365 business mail, and Microsoft may prioritize factors other than IP reputation.
Key opinions
- IP Reputation Limited: IP reputation has limited impact on deliverability, especially with large providers like Microsoft; Content and engagement are more important.
- Internal Settings: Incorrect SCL or BCL settings implemented by the admin team could be causing the problem.
- Multiple Reputations: Microsoft considers multiple reputations, not just IP: domain reputation and content reputation are also important.
- Sender Score Inapplicable: Sender score reputation is based on consumer domains; it has zero connection to the IP when sending business mail out of O365.
Key considerations
- Check Headers: Review full headers, including SCL and BCL levels, to diagnose configuration issues.
- Authentication: Ensure proper setup of SPF, DKIM, and DMARC records.
- Avoid Outlook for Marketing: Avoid using Outlook for marketing without proper opt-out mechanisms.
- Investigate Blocks: If facing delivery issues, investigate potential blocks by Microsoft.
Expert view
Expert from Email Geeks suggests checking SCL and BCL levels in the full headers to diagnose if the issue is a setting implemented by the admin team. It's unlikely to be an IP problem.
9 Dec 2022 - Email Geeks
Expert view
Expert from Email Geeks warns that going into spam at Microsoft suggests a potential block. If using Microsoft for outbound mail and facing delivery issues within Microsoft, there should be recourse for investigation. Also advises against using Outlook for marketing messages without proper opt-out methods.
27 Jul 2022 - Email Geeks
What the documentation says
4 technical articles
Emails sent via Microsoft SMTP might land in spam despite good Sender Score due to various factors outlined in official documentation. These factors include recipient mail server configurations, sophisticated spam filtering algorithms (like EOP), user-reported spam, email content, and missing/incorrect SPF, DKIM, and DMARC records. User feedback, captured via SmartScreen filter and feedback loops, significantly influences deliverability. If all best practices are followed, Microsoft recommends filling out a sender information form for investigation.
Key findings
- Multiple Factors: Deliverability is influenced by a combination of factors, including mail server configurations, spam filtering, user feedback, and content.
- Authentication Importance: Correct SPF, DKIM, and DMARC configuration is crucial for Microsoft to accept emails; misconfiguration can lead to spam classification.
- User Feedback Impact: Microsoft's SmartScreen filter relies heavily on user feedback; negative feedback (emails marked as junk) negatively impacts deliverability.
- Misclassification: Microsoft's EOP, despite using advanced anti-spam technologies, can sometimes misclassify legitimate email.
Key considerations
- Configure Authentication: Thoroughly configure and validate SPF, DKIM, and DMARC records to authenticate your sending domain.
- Monitor Feedback Loops: Actively monitor feedback loops (FBLs) and promptly address any user complaints to maintain a positive sender reputation.
- Review Email Content: Ensure email content adheres to best practices to avoid triggering spam filters.
- Submit Sender Information: If following all best practices and still experiencing deliverability issues, submit a sender information form to Microsoft for investigation.
Technical article
Documentation from RFC explains that SPF records, DKIM and DMARC records are important and need to be set up correctly in order to ensure Microsoft accepts your email. It may cause Microsoft to throw your emails to spam if not configured correctly.
16 May 2025 - RFC
Technical article
Documentation from SparkPost explains that Microsoft's SmartScreen filter heavily relies on user feedback. If enough users mark your emails as junk, your deliverability will suffer, even if your Sender Score is good elsewhere. They recommend actively monitoring feedback loops (FBLs) and promptly addressing any complaints.
17 Apr 2023 - SparkPost
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