Email has become the backbone of how we work, collaborate, and respond to urgent needs. Unfortunately, it’s also one of the most exploited gateways for cyberattacks. One particularly disruptive tactic on the rise is email bombing & Microsoft is now stepping up with a solution that runs silently, but powerfully, in the background. The new Mail Bombing Detection in Microsoft Defender for Office 365 is set to roll out globally between late June and late July 2025.
What is Email Bombing?
Email bombing isn’t just about annoying someone with spam. It’s strategic, disruptive, and often used as a smokescreen for more serious attacks. Here’s what typically happens:
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An attacker floods your inbox with hundreds or thousands of messages in a short window.
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Important emails like password reset links, security alerts, or financial transaction confirmations get buried.
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The overload can distract security teams, clog up automation workflows, and mask other threats slipping in at the same time.
It’s a tactic we’ve seen in high-profile attacks, and while it’s not new, the volume and sophistication are on the rise.
How Defender for Office 365 Now Handles It
Starting late June through late July 2025, Microsoft is rolling out Mail Bombing Detection across all tenants using Defender for Office 365.
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Uses AI and machine learning to detect high-volume, abnormal message patterns.
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Automatically identifies and flags email bombing campaigns.
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Moves the messages to the Junk folder, keeping the inbox clean and important emails visible.
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Respects Safe Senders policies, so trusted contacts aren’t mistakenly junked.
This means you get smarter, real-time protection, with no new rules or policies to configure.
Where You’ll See the Impact
Mail bombing events will be surfaced across your familiar security and compliance tools:
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Threat Explorer
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Email Entity View
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Email Summary Panel
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Advanced Hunting queries
These interfaces will now include mail bombing as a distinct detection type, making it easier to track and analyze these events within your environment.
Will Mail Bombing Detection Affect Audit Logs and Compliance Reports?
Because the feature changes how emails are processed and where they’re routed (to Junk), it might have implications for compliance and eDiscovery.
| Area | Impact |
|---|---|
| Email Routing | ✔️ Yes — messages redirected to Junk |
| AI/ML Usage | ✔️ Yes — new detection logic |
| eDiscovery & Audit Logs | ⚠️ Possibly impacted — flagged emails in Junk might affect visibility |
| Compliance Dashboards | ⚠️ New detection events may surface in reporting |
If your organization uses Microsoft Purview or has strict eDiscovery policies, test and validate how mail bombing detection impacts your compliance workflows.
Final Thoughts
The introduction of Mail Bombing Detection in Microsoft Defender for Office 365 marks another step forward in Microsoft’s commitment to proactive, intelligent threat defense. As email-based attacks evolve, so must our defenses and this feature provides exactly that: a hands-free, AI-powered safeguard against a growing and disruptive tactic.
