Microsoft Adds Mail Bombing Detection to Office 365 Security Suite

Cybersecurity threats are evolving, and one of the most disruptive tactics facing organizations is email bombing—a method where attackers flood a target’s inbox with thousands of messages in a short time.

The intent is to overwhelm systems, obscure legitimate communications (such as security alerts), and create chaos that can be exploited for further attacks, including phishing, credential theft, and malware deployment.

Attackers often leverage automated tools or botnets to generate these high volumes, and may follow up the chaos with social engineering, impersonating IT support to trick users into granting remote access.

Technical Innovation:

To combat this rising threat, Microsoft Defender for Office 365 has introduced a dedicated Mail Bombing Detection feature, rolling out globally from late June to early July 2025.

This new capability is enabled by default and requires no manual configuration, providing seamless protection for all users.

The detection technology operates by:

• Intelligently tracking message volumes from various sources and over specific time intervals.
• Leveraging historical sender patterns and spam content signals to distinguish between legitimate and malicious email surges.
• Automatically classifying suspected email bombing messages and routing them to the Junk folder in Outlook, ensuring that vital communications remain accessible.
• Respecting Safe Senders lists, so trusted emails are not incorrectly filtered, minimizing false positives.

Security teams can monitor and respond to these attacks through new detection types labeled “Mail Bombing” in Threat Explorer, Email Entity View, Email Summary Panel, and Advanced Hunting within the Defender for Office 365 interface.

SOC analysts can also create custom detection rules and track the frequency and volume of attacks for proactive defense.

Compliance and Operational Impact:

The introduction of Mail Bombing Detection brings several considerations for security operations and compliance:

• Data Processing Changes: The feature modifies how email messages are classified and routed, potentially impacting audit logging and eDiscovery visibility for emails sent to the Junk folder.
• AI and Machine Learning: New detection logic is powered by advanced AI/ML models, enhancing the accuracy and speed of threat identification.
• Visibility and Reporting: Security Operations Analysts and Administrators will see new detection types in compliance dashboards, aiding in incident response and compliance monitoring.

Recommended Actions for Organizations:

• Inform Security Operations teams about the new detection capability.
• Update internal documentation and training materials to reflect the change.
• Review Junk folder handling policies to ensure alignment with organizational needs.

By automating the detection and mitigation of email bombing attacks, Microsoft Defender for Office 365 strengthens organizations’ defenses against a sophisticated and increasingly common threat, empowering security teams to maintain focus on genuine risks.

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