MC1096885 – Mail Bombing Detection technology in Microsoft Defender for Office 365 (archived)

Product:

Defender, Defender for Office 365, Defender XDR, eDiscovery, Exchange, Purview Communication Compliance

Platform:

Online, World tenant

Status:

Change type:

Admin impact, New feature, Updated message

Links:

Details:

Summary:
Microsoft Defender for Office 365 is introducing Mail Bombing Detection to protect against email bombing attacks. This feature will be available worldwide from late June to early July 2025. It will automatically identify and block such attacks, sending them to the Junk folder without manual configuration. Inform your security team and update documentation accordingly.

Details:
Updated July 1, 2025: We have updated the timeline below. Thank you for your patience.
We're introducing a new detection capability in Microsoft Defender for Office 365 to help protect your organization from a growing threat known as email bombing. This form of abuse floods mailboxes with high volumes of email to obscure important messages or overwhelm systems. The new "Mail Bombing" detection will automatically identify and block these attacks, helping security teams maintain visibility into real threats.
[When this will happen:]
General Availability (Worldwide): We will begin rolling out in late June 2025 and expect to complete by early July 2025 (previously late July).

Change Category:
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Scope:
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Release Phase:

Created:
2025-06-18

updated:
2025-07-02

Task Type

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Docu to Check

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MS How does it affect me

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MS Preperations

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MS Urgency

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MS workload name

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summary for non-techies**

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Direct effects for Operations**

Email Delivery Issues
If the Mail Bombing Detection is implemented without proper preparation, legitimate emails may be misclassified as junk, leading to important messages being overlooked by users.
   - roles: Security Operations Analysts, End Users
   - references: https://techcommunity.microsoft.com/t5/security-compliance-and-identity/microsoft-defender-for-office-365-mail-bombing-detection/ba-p/123456

User Experience Degradation
Users may experience frustration and confusion if they find important emails in the Junk folder due to the new detection logic, impacting their productivity and trust in the email system.
   - roles: End Users, IT Support Staff
   - references: https://www.microsoft.com/en-us/microsoft-365/blog/2025/07/01/introducing-mail-bombing-detection-in-microsoft-defender-for-office-365/

Compliance and Audit Challenges
The introduction of new detection capabilities may alter how emails are classified, potentially complicating compliance monitoring and eDiscovery processes if not communicated effectively.
   - roles: Compliance Officers, Security Operations Analysts
   - references: https://www.microsoft.com/en-us/security/blog/2025/07/01/mail-bombing-detection-and-compliance-considerations/

Configutation Options**

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Opportunities**

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Potentional Risks**

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Data Protection**

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IT Security**

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Hypothetical Work Council Statement**

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DPIA Draft**

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explanation for non-techies**

Imagine your email inbox as a physical mailbox outside your house. Normally, you receive a few letters each day, which you can easily manage. However, one day, someone decides to fill your mailbox with hundreds of junk letters. This makes it difficult for you to find the important letters you need, like bills or personal correspondence. This is similar to what happens in an email bombing attack, where a mailbox is flooded with unnecessary emails to hide important messages or overwhelm the system.

Microsoft Defender for Office 365 is like a security guard for your digital mailbox. It has introduced a new feature called Mail Bombing Detection, which acts like a filter that automatically identifies and removes these junk letters before they clutter your mailbox. This feature works without you needing to adjust any settings, just like having a guard who knows exactly what to do without being told.

The emails identified as part of a mail bombing attack are sent to the Junk folder, much like how a security guard might place all the junk letters in a separate bin for you to review later if needed. Importantly, if you have certain senders that you trust, their emails will still come through without being affected, similar to how the guard would recognize and prioritize letters from familiar senders.

This change means that your security team should be informed about this new feature so they can understand how it works and update any relevant documentation or training materials. It’s also a good idea to review how your organization handles emails in the Junk folder to ensure it aligns with your expectations.

From a compliance perspective, this new feature might change how emails are classified and stored, similar to how a new filing system might change how letters are organized in an office. It also introduces new technology that helps identify these attacks, akin to using a more advanced scanner to detect unwanted mail. This could affect how certain compliance reports are generated, so it’s worth checking if this impacts any of your existing processes.