How a New URL Detection Rule in Exchange Online is Blocking Real Best Emails as Phishing - FeedHours

How a New URL Detection Rule in Exchange Online is Blocking Real Emails as Phishing

Microsoft Exchange Online, a key part of Microsoft 365, is currently experiencing a significant disruption affecting email delivery for businesses and individual users. The cause? A new URL detection rule that is incorrectly flagging legitimate emails as phishing attempts. This issue, which began on February 5, 2026, has prevented many users from sending or receiving emails, creating confusion and operational delays for organizations worldwide.

The Root Cause: Overly Aggressive URL Detection

Microsoft confirmed that the problem stems from a recently introduced URL rule designed to catch more sophisticated phishing and spam emails. While the intention was to strengthen Exchange Online’s security, the update has instead misclassified safe URLs as malicious.

“We’ve determined that the URLs associated with these email messages are incorrectly marked as phish and quarantined in Exchange Online due to ever-evolving criteria aimed at identifying suspicious email messages, as spam and phishing techniques have become more sophisticated in avoiding detection,” Microsoft said in a service alert.

The updated rule was intended to enhance phishing detection capabilities, but it overreached, resulting in a surge of false positives that affected normal email flow.

User Impact: Emails Held Hostage

The effects of this issue have been felt across Exchange Online:

Quarantined Emails: Legitimate messages are being routed to quarantine instead of users’ inboxes.
Communication Delays: Many users are unable to send or receive important emails, which can disrupt business operations.
Pending Resolution: Microsoft is actively working to release quarantined emails and unblock legitimate URLs.

“We’re reviewing the release of quarantined messages for affected users and working on confirming legitimate URLs are unblocked,” Microsoft stated. Some users may now see previously flagged emails delivered as remediation progresses.

While Microsoft has not disclosed the total number of affected users or impacted regions, the issue has been classified as a service incident, signaling noticeable impact for Exchange Online customers.

Timeline of the Issue

February 5, 2026: The problem began, with users reporting quarantined emails and delivery failures.
Over the Weekend: Microsoft confirmed that the new URL detection rule is the source of the false positives.
Ongoing: Engineers continue to review quarantined messages and adjust detection criteria to restore normal operations.

Historical Context: Not the First Time

Exchange Online has experienced similar false positive incidents in the past, highlighting the challenges of balancing security with email delivery:

March 2025: Anti-spam systems mistakenly quarantined emails from legitimate users.
May 2025: Machine learning models incorrectly flagged Gmail emails as spam.
September 2025: Anti-spam service bugs blocked URLs and quarantined emails in Exchange Online and Microsoft Teams.

These incidents illustrate the ongoing challenge Microsoft faces in detecting phishing emails without impacting legitimate communication.

Recommendations for Users and Admins

While Microsoft works on a full resolution, users and administrators can take steps to mitigate the impact:

Check Quarantine Folders: Regularly review quarantined emails to identify legitimate messages.
Release Legitimate Emails: Manually release emails that were incorrectly flagged to ensure timely communication.
Notify Stakeholders: Inform colleagues, clients, and partners about possible email delays.
Monitor Microsoft Updates: Stay informed via the Microsoft 365 Service Health Dashboard for remediation updates.

These measures will help maintain smooth communication until the issue is fully resolved.

Key Takeaways

The Exchange Online incident underscores the delicate balance between robust phishing protection and uninterrupted email delivery. While URL detection rules are critical for security, overzealous rules can lead to false positives, disrupting businesses and individual users alike. Microsoft is actively remediating the issue by releasing quarantined emails and adjusting detection criteria.

Users are advised to remain vigilant, check their quarantines, and communicate with stakeholders about potential email delivery delays until full remediation is achieved.

Faq

1. Why is Microsoft Exchange Online flagging legitimate emails as phishing?

Microsoft recently implemented a new URL detection rule to enhance phishing and spam protection. However, this rule is incorrectly marking some safe URLs as malicious, resulting in legitimate emails being quarantined.

2. When did this issue start affecting users?

The incident began on February 5, 2026, and continues to impact some Exchange Online users worldwide.

3. Who is affected by this Exchange Online issue?

While Microsoft has not disclosed exact numbers, it affects Exchange Online customers across multiple regions. Any user receiving emails with URLs flagged by the new rule could be impacted.

4. Can users still send or receive emails?

Some users may experience delays in sending or receiving emails. Emails identified as phishing are routed to quarantine until Microsoft resolves the issue.

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