How does Microsoft Office 365 filter or block emails based on URL reputation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 31 Jul 2025
Updated 16 Aug 2025
8 min read
Email deliverability to Microsoft Office 365 (O365) recipients can often feel like navigating a complex maze. One of the critical factors determining whether your emails land in the inbox, junk, or are outright blocked is URL reputation. Microsoft employs sophisticated filtering mechanisms, particularly within Defender for Office 365, to assess the trustworthiness of URLs contained within emails.
This goes beyond just checking the sender's IP or domain reputation. Even if your sending infrastructure is pristine, a single questionable URL in your email content can trigger O365 filters, leading to deliverability issues. It's a common challenge for email marketers and system administrators alike, often resulting in emails landing in spam folders.
Understanding how Microsoft evaluates URL reputation is key to ensuring your legitimate communications reach their intended recipients. It involves a multi-layered approach that constantly adapts to new threats, making it a dynamic and sometimes unpredictable factor in email deliverability.
How URL reputation is assessed
Microsoft Office 365, through its various security layers, primarily Exchange Online Protection (EOP) and Defender for Office 365, employs a comprehensive system to assess URL reputation. This system is designed to identify and block malicious links associated with phishing, malware, or other cyber threats. It considers several factors to build a profile for each URL.
One primary method is the use of extensive URL blocklists (also known as blacklists). Microsoft maintains and subscribes to large databases of known bad URLs. These lists are continuously updated through various threat intelligence feeds, user submissions, and automated detection systems. If a URL in an incoming email matches an entry on one of these lists, it's immediately flagged as suspicious or malicious. Microsoft utilizes a vast list of domains that are known to send spam.
Beyond static blocklists (or blacklists), O365 also performs real-time analysis. This dynamic assessment considers factors like the age of the domain, its hosting location, the presence of suspicious redirects, and behavioral patterns. For instance, new domains or those hosted in historically problematic regions might receive higher scrutiny. The system also checks if the URL leads to a known phishing site or malware distribution point.
Furthermore, user feedback plays a significant role. When users report emails as junk or phishing, the URLs within those emails contribute to their negative reputation. Conversely, legitimate URLs that are frequently clicked or associated with positive user engagement can build a positive reputation over time. This continuous feedback loop helps Microsoft adapt its filters to emerging threats and legitimate sending patterns.
The role of Safe Links
A cornerstone of Microsoft's URL reputation filtering is Safe Links, a feature within Microsoft Defender for Office 365. Safe Links provides time-of-click verification of URLs in email messages and other Microsoft 365 apps. This means that when a user clicks a link in an email, Safe Links intercepts the click and scans the URL in real-time to determine if it's malicious.
When Safe Links is enabled, all URLs in incoming emails are rewritten (or wrapped) to point to a Microsoft Safe Links server. This redirection allows Microsoft to perform its real-time checks. If the URL is determined to be safe, the user is redirected to the original destination. If it's malicious, a warning page is displayed, or access to the site is blocked entirely. This protects users even if a URL's reputation changes after the email has been delivered.
Safe Links policies can be configured by administrators to apply to specific users, groups, or domains, allowing for granular control over how URLs are processed. The system also learns from user behavior and global threat intelligence to continuously update its understanding of URL safety. This proactive scanning means that Microsoft may scan links in your emails at a high rate, impacting deliverability if not configured correctly.
Example Safe Links policy configuration (PowerShell)Powershell
Based on a URL's reputation, Microsoft Office 365 can take various actions, ranging from mild filtering to outright blocking. The severity of the action depends on the perceived threat level of the URL and the specific anti-spam and anti-phishing policies configured for the recipient's tenant.
For URLs with a low but not critical reputation, emails might be sent to the recipient's junk folder. This is a common scenario when emails contain URLs that Microsoft deems suspicious due to content, domain age, or unusual patterns, but not overtly malicious. Many legitimate emails can inadvertently trigger this. This is why emails can end up in junk folders.
In cases of a moderate threat, such as suspected phishing or malware, emails might be quarantined. When an email is quarantined, it's held in a secure location, and the recipient usually receives a notification allowing them to review the message and decide whether to release it or report it as a false positive. This gives administrators an opportunity to fix issues when emails are quarantined.
For definitively malicious URLs, such as those leading to known phishing sites or malware downloads, O365 will outright block the email or strip the malicious URL from the message. This ensures the highest level of protection for end-users. Microsoft's anti-spam policies can be configured to allow or block senders, but URL reputation is often an overriding factor.
Over-filtering consideration
While Microsoft's stringent URL filtering is crucial for security, it can sometimes lead to legitimate emails being flagged. If your emails containing valid URLs are consistently being blocked or quarantined, it's important to understand the specific policy settings that may be causing this behavior within the recipient's Office 365 tenant. This may require collaboration with the recipient's IT team.
Troubleshooting URL reputation issues
If you're experiencing deliverability issues with Office 365 recipients and suspect URL reputation is the culprit, there are several steps you can take to troubleshoot. The first step is to isolate the problematic URL. This often involves sending test emails with and without the suspected URL to O365 test accounts to see if the deliverability improves.
Ensure that any URLs you include in your emails are hosted on reputable domains and are free from any content that could be perceived as suspicious. Avoid using URL shorteners indiscriminately, as these can sometimes mask the true destination and raise red flags with filters. If you must use them, ensure they are from well-known, trusted providers.
For persistent issues, especially if your URLs are legitimate but still being flagged, you might need to engage with the recipient's Office 365 administrator. They have access to the Tenant Allow/Block List in Microsoft Defender where they can explicitly allow (safelist) specific URLs or domains. This manual override can help ensure your emails bypass some of the automated filtering. In some cases, you may need to submit a request to Microsoft to delist your IP or domain.
Problematic URLs
Untrusted domains: Links to domains with poor historical reputation, known for spam or phishing.
Malicious content: URLs leading to sites hosting malware, ransomware, or deceptive content.
Spam traps: Links that lead to domains or pages identified as spam traps.
Blacklisted (blocklisted) URLs: URLs explicitly listed on Microsoft's internal blocklists or external real-time blocklists.
Solutions and best practices
Domain authentication: Implement strong email authentication like SPF, DKIM, and DMARC for your sending domain.
Reputable hosting: Host your linked content on domains with a good reputation history.
Monitor URL reputation: Regularly check the reputation of URLs used in your email campaigns.
Clear content: Ensure landing page content is legitimate and matches the email's context.
Views from the trenches
Best practices
Always use full, canonical URLs in your emails, avoiding unnecessary redirects or URL shorteners where possible.
Regularly scan all links within your email content for potential malware or phishing indicators before sending campaigns.
Host your landing pages and content on well-maintained domains with established, positive reputations.
Monitor your engagement rates, as high click-through rates on legitimate links can positively influence URL reputation.
Implement DMARC for your sending domains to enhance authentication and provide visibility into email streams.
Common pitfalls
Using generic or untrustworthy URL shorteners that are frequently abused by spammers and phishers.
Linking to compromised websites or pages that have been infected with malicious code.
Including URLs in emails that lead to content that is inconsistent with your brand or sender identity.
Neglecting to monitor your domain and IP reputation, allowing it to degrade unnoticed by Microsoft filters.
Sending emails with an excessive number of links, which can appear suspicious to O365 filters.
Expert tips
Test your emails with the actual links on various O365 accounts to see how they render and if they are filtered.
Encourage recipients experiencing issues to add your sending domain to their safe sender list in Office 365.
Review Microsoft's official documentation for anti-spam and Safe Links policies for detailed insights.
Consider setting up a dedicated subdomain for links to track their reputation separately from your main domain.
Actively manage your sender reputation through consistent, high-quality email practices to build trust with Microsoft.
Marketer view
Marketer from Email Geeks says they have seen sporadic filtration and quarantine reports from Office 365 that seemed to be linked to a common URL within the emails. Troubleshooting these issues is challenging because O365 behavior heavily depends on individual tenant configurations.
2023-01-18 - Email Geeks
Expert view
Expert from Email Geeks says that Microsoft is known to filter or block emails based on the presence of low-reputation URLs. They recommend sending test emails with and without the suspected URL to different test accounts to pinpoint the issue.
2023-01-18 - Email Geeks
Conclusion
Microsoft Office 365's reliance on URL reputation as a filtering mechanism is a powerful defense against email-borne threats. For email senders, this means that merely having good sender reputation for your IP and domain is not enough. The reputation of every single URL you include in your emails is under scrutiny.
To maintain high deliverability to O365 inboxes, prioritize using clean, reputable URLs and actively monitor their standing. Regular testing, adherence to email best practices, and understanding Microsoft's filtering nuances are essential. This proactive approach will help ensure your messages, along with their embedded links, are seen as trustworthy and reach their intended audience.
Microsoft Office 365 (O365) recipients can often feel like navigating a complex maze. One of the critical factors determining whether your emails land in the inbox, junk, or are outright blocked is URL reputation. Microsoft employs sophisticated filtering mechanisms, particularly within Defender for Office 365, to assess the trustworthiness of URLs contained within emails.
