Microsoft Enhances Office 365 Defender to Stop Email Bombing Campaigns

Microsoft has announced a significant enhancement to its Office 365 Defender suite with the introduction of Mail Bombing Detection, a new feature designed to combat the rising threat of email bombing attacks.

This capability will be rolled out globally, starting in late June 2025, and is expected to reach all customers by late July 2025.

Addressing the Email Bombing Threat

Email bombing is a disruptive tactic where attackers flood mailboxes with massive volumes of emails, aiming to obscure important messages or overwhelm organizational systems.

This can hinder business operations, mask genuine threats, and create significant challenges for security teams. 

Microsoft’s new Mail Bombing Detection feature is engineered to automatically identify and block such attacks, ensuring that security teams maintain visibility into real threats without being distracted by noise.

The Mail Bombing Detection feature is enabled by default and requires no manual configuration, making it easy for organizations to benefit from enhanced protection without additional administrative overhead. 

Once active, the detection will be visible in several key security tools within the Microsoft Defender for Office 365 ecosystem, including:

• Threat Explorer
• Email Entity View
• Email Summary Panel
• Advanced Hunting

Messages identified as part of a mail bombing campaign will be automatically routed to the Junk folder.

Importantly, Safe Senders settings will continue to be honored, ensuring that legitimate communications are not inadvertently impacted.

Impact on Security Operations

Security Operations Analysts and Administrators will notice a new detection type labeled “Mail Bombing” in the aforementioned tools.

This visibility allows for rapid investigation and response, leveraging Microsoft’s advanced threat intelligence and AI-driven detection logic. 

The feature’s integration with existing workflows means organizations can maintain a streamlined approach to email security while benefiting from the latest advancements in threat detection.

The introduction of Mail Bombing Detection alters how email messages are classified and routed, as it modifies the processing and storage of existing data.

It also introduces new AI and machine learning capabilities to enhance detection accuracy.

Organizations should be aware that this may impact audit logging or eDiscovery visibility for messages sent to the Junk folder, and new detection types may appear in compliance dashboards.

To prepare, Microsoft recommends that organizations:

• Inform Security Operations teams about the new detection capability
• Update internal documentation and training materials
• Review Junk folder handling policies to ensure they align with organizational expectations

As email-based threats continue to evolve, Microsoft’s proactive enhancements to Defender for Office 365 demonstrate a commitment to providing robust, automated protection for organizations worldwide.

The Mail Bombing Detection feature is a testament to Microsoft’s ongoing investment in AI-driven security and its focus on empowering security teams to stay ahead of emerging threats.

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