How does Microsoft Office 365 filter or block emails based on URL reputation?

Key findings

• Reputation-based filtering: Microsoft 365 heavily relies on URL reputation to identify and filter emails. If a URL within an email has a low or compromised reputation, it can trigger filtering actions, even if the sender's IP or domain reputation is otherwise good.
• Dynamic assessment: The reputation of URLs is constantly assessed based on various factors, including known malicious sites, suspicious patterns, and user feedback. This dynamic assessment means that a URL's status can change over time.
• Tenant-specific configurations: Email filtering behavior in Office 365 can vary significantly based on individual tenant configurations. Administrators have options to customize spam and threat policies, which can impact how URLs are handled.
• Impact on deliverability: Emails containing low-reputation URLs might be quarantined, moved to junk folders, or rejected. This can lead to significant deliverability issues for legitimate senders, as detailed in our guide on why emails land in Office 365 spam folders.

Key considerations

• URL shorteners: The use of URL shorteners can impact deliverability if the shortened domain itself has a poor reputation or is frequently used for malicious purposes. More on this can be found in our article on how URL shorteners and domain reputation impact email deliverability.
• Testing with care: Before sending large campaigns, test emails with the suspicious URL to multiple Office 365 accounts to observe filtering behavior. This can help isolate whether the URL is the cause of deliverability issues.
• Microsoft's stance on support: Microsoft is generally responsive to outright block issues via their delisting form at sender.office.com, but less so for non-block issues like sporadic filtering or quarantining.
• Recipient action: Sometimes, end-recipients can reach out to their IT administrators or Microsoft directly to have a URL reclassified if it's erroneously marked as low reputation.

Key opinions

• Troubleshooting difficulty: Marketers frequently express frustration over the challenges of troubleshooting Office 365 filtering, particularly when it's not an outright block but rather sporadic quarantines or spam folder placement.
• URL as a primary culprit: Many suspect that a specific URL within their email content is the root cause of these filtering issues, leading them to isolate and test URLs for reputation. This is especially true for emails sent via third-party ESPs.
• Inconsistent behavior: The filtering behavior can seem inconsistent, often depending on the specific Office 365 tenant's configuration rather than a global rule.
• Limited support for non-blocks: There's a general consensus that Microsoft's support is less receptive to inquiries about non-block related filtering issues, making self-diagnosis and mitigation crucial. More insights on general spam filtering can be found in our article why your emails are going to spam.

Key considerations

• Proactive URL reputation management: Regularly monitor the reputation of domains and URLs used in your emails, especially for links to landing pages, images, or tracking pixels. Address any reputation issues promptly.
• Split testing: When facing issues, marketers should consider sending test emails with and without the suspected problematic URL to isolate its impact on Office 365 filtering.
• Engaging recipients: Encourage Office 365 recipients who encounter false positives to report them to Microsoft or their IT department. This 'customer-side' feedback can sometimes lead to URL reclassification.
• Admin portal access: If possible, work with your IT team or client's IT team to review the Microsoft 365 Defender admin portal settings related to anti-spam and anti-phishing policies, as these directly influence URL filtering. Learn more about configuring these policies from the AdminDroid Blog.

Key opinions

• Behavioral analysis: Microsoft's filters don't just rely on static blacklists, but also analyze user interactions and historical data to build URL reputation. This includes clicks on malicious links or reports of phishing associated with a URL.
• Link wrapping: Office 365 often rewrites or 'wraps' URLs in emails to scan them at the time of click. If a URL is deemed malicious at that point, the user is blocked from accessing it, even if the email initially passed filters.
• Layered filtering: URL reputation is one layer of a complex filtering system that includes IP reputation, sender domain reputation, content analysis, and user feedback. All these factors contribute to the final delivery decision. This ties into how Microsoft handles IP reputation.
• Brand indicators: Positive brand indicators and consistent email authentication (like DMARC) can indirectly boost the perceived trustworthiness of URLs from a sending domain, even if the URL itself doesn't have an established individual reputation. Understanding your email domain reputation is essential.

Key considerations

• Dedicated domains for links: Some experts recommend using separate subdomains or even distinct domains for link tracking and content URLs, allowing for better reputation management specific to different types of links.
• Monitor blocklists: While not directly URL-based, being listed on major IP or domain blacklists can negatively impact the perception of any URLs contained in your emails. Regular blocklist monitoring is advisable.
• URL auditing: Periodically audit all URLs included in your email campaigns, checking for potential redirects, compromised links, or unintended associations that could hurt their reputation.
• Understand defender policies: Familiarize yourself with Microsoft Defender for Office 365 policies, specifically those related to Safe Links and Safe Attachments, as they are central to URL protection. More details on spam detection reports can be found on AdminDroid.

Key findings

• Safe links: Microsoft Defender for Office 365 includes a Safe Links feature that proactively scans URLs in incoming emails. This feature rewrites URLs and checks them at the time of click to ensure they are safe.
• Anti-phishing policies: These policies are designed to detect and block phishing attempts, which often rely on malicious URLs. They can flag or block emails containing suspicious links, even if the sender's domain passes other checks.
• Bulk complaint level (BCL): The BCL helps organizations determine how strictly to filter bulk emails. While primarily for bulk, a high BCL can lead to stricter URL filtering due to the overall perception of the sending campaign.
• General filter rules: Microsoft 365 uses predefined rules and algorithms to detect and block known spam patterns and common phishing attempts, which often involve assessing URL legitimacy and reputation.

Key considerations

• Policy configuration: Administrators can configure anti-spam and anti-phishing policies in the Microsoft 365 Defender portal to adjust sensitivity levels for URL filtering, allowing for custom allow or block lists for specific URLs or domains. This is part of complying with Outlook's new sender requirements.
• User overrides: Microsoft 365 allows for overrides or exceptions to be set, enabling certain emails or URLs to bypass standard security filters. This is often used for important internal communications or trusted external senders. More on mastering overrides can be found on Microsoft TechCommunity.
• Reporting: Users can report emails as junk, phishing, or not junk, which feeds into Microsoft's reputation systems, including those for URLs. This crowdsourced feedback helps refine filtering accuracy.
• Authentication standards: Proper implementation of email authentication standards like SPF, DKIM, and DMARC is crucial, as Microsoft relies on these to verify sender legitimacy, which in turn enhances the trustworthiness of URLs originating from those senders. Our guide on advanced email authentication provides further detail.