Why are emails being filtered by Microsoft and ending up in junk folders?
Michael Ko
Co-founder & CEO, Suped
Published 6 Jun 2025
Updated 18 Aug 2025
8 min read
It can be frustrating when emails you send consistently land in recipients' junk or spam folders, especially when you're sure they're legitimate. This issue is particularly common with Microsoft email services, including Outlook.com and Office 365. Understanding why this happens requires delving into how Microsoft's sophisticated filtering systems operate and what factors they prioritize when assessing incoming mail. My goal is to shed light on these mechanisms and offer practical strategies to improve your email deliverability.
Understanding Microsoft's filtering mechanisms
Microsoft's email filtering system is designed to protect users from unwanted mail, including spam, phishing attempts, and malware. Unlike simpler filters, Microsoft employs a complex set of algorithms and reputation metrics to assign a score to every incoming message. Two crucial scores are the Spam Confidence Level (SCL) and the Bulk Complaint Level (BCL). A high SCL indicates a strong likelihood that an email is spam, while a high BCL suggests that many recipients have marked similar bulk emails as junk.
The reputation of your sending IP address and domain plays a significant role in this scoring. Even if you're sending from a dedicated IP, a sudden increase in volume, a spike in complaints, or a history of low engagement can negatively impact your sender reputation, causing your emails to be filtered more aggressively. Microsoft closely monitors these behaviors, and a poor reputation can lead to emails being sent directly to junk, or even quarantined.
Content analysis is another critical layer. Microsoft's filters scrutinize email content for characteristics often associated with spam, such as suspicious keywords, excessive capitalization, image-to-text ratio, and problematic links or attachments. This also includes hidden headers like X-Microsoft-Antispam-Mailbox-Tag which can influence delivery. It's a continuous learning process, and filters adapt based on global spam trends and user feedback.
Email authentication protocols are foundational to establishing trust with Microsoft's filters. SPF, DKIM, and DMARC records provide verifiable proof that your emails are legitimate and sent from authorized sources. Without these in place, or if they are misconfigured, your emails are far more likely to be flagged as suspicious and routed to the junk folder. This is a common reason why authenticated emails still go to junk.
Common culprits behind junk placement
Several factors can trigger Microsoft's spam filters, leading to your emails missing the inbox. One of the primary culprits is low sender reputation, which can be caused by a high complaint rate, a significant number of bounces, or sending to inactive or invalid email addresses. Even if individual recipients mark your emails as 'not junk,' a cumulative negative signal across many users can still push your overall reputation down. Many legitimate emails are caught because of this. You can learn more about this by reading this Microsoft Learn article.
Poor list hygiene is another major contributor. Sending emails to old, disengaged, or non-existent addresses can lead to increased bounce rates and hits on spam traps. Spam traps are email addresses specifically set up by ISPs (including Microsoft) to identify senders of unsolicited mail. Hitting a spam trap is a strong negative signal that can quickly lead to your IP or domain being added to internal (private) blocklists, drastically affecting your deliverability to Office 365 spam folders.
The content and formatting of your emails can also be problematic. Emails with too many images and too little text, embedded forms, shortened URLs, or an overabundance of promotional language are more likely to be flagged. Even certain fonts or colors can sometimes contribute to a higher spam score. Using common spam trigger words, whether intentionally or unintentionally, can significantly increase your SCL and the likelihood of junk folder placement. This is particularly true for automated templated emails.
A critical, yet often overlooked, factor is the Spam Confidence Level (SCL) and Bulk Complaint Level (BCL) thresholds within Microsoft's filtering system. These scores directly influence where your emails land. As Spotler highlights, if your emails consistently receive high SCL or BCL scores, they are almost guaranteed to end up in the junk or quarantine folders, regardless of other positive signals. This often indicates underlying issues with your sending practices or recipient engagement. You can learn more about how to improve these scores, especially your BCL score, by reading our guide to BCL scores.
Proactive strategies for improved inbox delivery
To improve your deliverability to Microsoft inboxes, start by ensuring your email authentication protocols are perfectly set up and aligned. This includes SPF, DKIM, and DMARC. Regularly check your DMARC reports for authentication failures or misconfigurations. Microsoft places a high emphasis on authenticated mail, and correct setup is a baseline requirement for good inbox placement. It's also worth checking for hidden SPF DNS timeouts that can affect Microsoft delivery.
Best practices
Strong authentication: Implement and monitor SPF, DKIM, and DMARC records to prove sender legitimacy.
Engagement focus: Send relevant emails that recipients want to open and click on, improving positive interactions.
List hygiene: Regularly clean your email lists to remove inactive or invalid addresses, reducing bounces and spam trap hits.
Actively manage your sender reputation. Monitor your complaint rates, bounce rates, and user engagement metrics. A high complaint rate is a clear signal to Microsoft that your emails are unwelcome. Encourage recipients to add you to their safe senders list and avoid marking your emails as junk. If you notice a sudden drop in deliverability to Microsoft addresses, investigate recent changes in your sending practices or content.
Optimize your email content. Avoid characteristics commonly associated with spam, such as excessive use of all caps, exclamation marks, or overly promotional language. Maintain a healthy balance between images and text, and ensure all links are legitimate and not shortened unnecessarily. A clean, well-formatted email is less likely to be flagged by Microsoft's content filters. Pay close attention to why your emails go to spam and how to fix it broadly.
Advanced troubleshooting and continuous monitoring
Microsoft (Outlook.com) provides resources for senders to monitor their sending reputation and identify issues. While not as comprehensive as some other postmaster tools, utilizing available tools and monitoring your email program's key metrics consistently is essential. Pay attention to any bounce messages that include specific error codes from Microsoft, as these can provide clues about the underlying problem. Sometimes, the problem could simply be due to Microsoft's filter changes.
Gradual IP warming for new IPs or domains is critical, even with clean lists. Microsoft, like other ISPs, prefers to see consistent, gradual sending volume increases rather than sudden spikes. This helps establish a positive sending history and reputation over time, reducing the likelihood of your emails being flagged as suspicious. For those transitioning to Microsoft 365 or Google Workspace, addressing common DMARC issues can also prevent junking.
Finally, ensure you have a clear and prominent unsubscribe option in every marketing email. A high rate of spam complaints often stems from recipients being unable to easily opt out of your mailing list. Making the unsubscribe process simple and transparent can significantly reduce complaints, which in turn improves your sender reputation with Microsoft and helps keep your emails out of the junk folder.
Views from the trenches
Best practices
Maintain meticulous list hygiene, regularly removing unengaged subscribers and invalid addresses to reduce bounces and spam trap hits.
Always include a prominent and easy-to-use unsubscribe link in every email to minimize spam complaints.
Ramp up email volume gradually for new IPs or domains to build a strong sending reputation with Microsoft's filters.
Common pitfalls
Ignoring DMARC reports, which often contain critical information about authentication failures and potential junking issues.
Sending emails with an excessive image-to-text ratio or overly promotional content, triggering spam filters.
Failing to monitor and react to negative sender reputation signals, such as high bounce rates or spam complaints.
Expert tips
Use Microsoft's Postmaster Tools to gain insights into your sender reputation and deliverability performance.
Segment your audience and tailor content for higher engagement, reducing the likelihood of emails being marked as junk.
Implement a double opt-in process for new subscribers to ensure high-quality, engaged recipients from the start.
Marketer view
Marketer from Email Geeks says they have noticed Microsoft filtering issues becoming more frequent, impacting dedicated IPs.
2020-01-17 - Email Geeks
Marketer view
Marketer from Email Geeks says they've observed a general change in overall inbox placement with Microsoft.
2020-01-17 - Email Geeks
Key takeaways for Microsoft deliverability
Navigating Microsoft's email filters can be challenging, but a proactive and systematic approach can significantly improve your inbox placement. By prioritizing strong authentication, maintaining a healthy sender reputation, optimizing your content, and meticulously managing your email lists, you can build trust with Microsoft's systems and ensure your legitimate emails reach their intended recipients' inboxes, rather than their junk folders.
