Most organizations running a third-party secure email gateway (SEG) alongside Microsoft 365 think they have defense in depth. They don't. They have redundancy — two layers inspecting the same signals, leaving the same gaps against the attacks that matter most.

Most organizations running a third-party SEG alongside Microsoft 365 have the second thing, not the first. As Microsoft 365 adoption grew, many teams kept their SEG as the primary email defense, disabling parts of Microsoft's native security stack in favor of familiar third-party tools. The result is overlapping controls that duplicate baseline protections while leaving the same gaps against advanced attacks.

By contrast, Microsoft 365 and Abnormal create complementary layers of protection. Microsoft delivers strong built-in defense against the vast majority of high-volume, known threats, while Abnormal adds behavioral AI to detect socially engineered and identity-based attacks that traditional inspection models often miss. That distinction matters because many of today’s most damaging incidents are low-volume and highly targeted, with social engineering remaining one of the most common breach patterns in the Verizon 2026 DBIR. Together, Microsoft and Abnormal provide broader coverage without duplicating the same controls.

The original business case for SEGs was clear. When enterprise email ran on-premises, organizations needed a dedicated perimeter layer for spam filtering, anti-malware, URL inspection, attachment analysis, and policy enforcement.

That gap has largely closed. Microsoft 365 and Google Workspace now deliver most of the core protections SEGs were built to provide, including spam and malware defense. Organizations that keep a legacy SEG are often paying twice for overlapping email security, while legacy SEGs often require routing mail around Microsoft's native controls to function. In many SEG displacement evaluations, teams find that most of what their SEG provides already exists in Microsoft 365.

The real gap is no longer basic email hygiene. It is advanced threat detection.

Third-party SEGs were built for a threat model centered on known bad indicators: threat intelligence, static definitions, and sandbox-based payload analysis. Those methods still stop many conventional attacks, but they are far less effective against modern, AI-enabled social engineering. Today’s attacks increasingly rely on trusted platforms, valid accounts, and technically clean, highly personalized messages that can bypass static inspection at scale. As a result, threats such as business email compromise, vendor fraud, and account takeover often appear benign because they are text-based, highly tailored, and grounded in real business context.

These threats are also among the most financially damaging, with average losses of approximately $123,000 per BEC incident. Microsoft's 2025 Digital Defense Report found that AI-driven phishing is now four times more effective than traditional campaigns — a sign of how quickly attacker tradecraft is outpacing legacy detection models. When a message comes from a compromised partner, references a legitimate workflow, and passes technical checks, both Microsoft’s native controls and legacy SEGs can miss the malicious intent behind it.

As Gartner® notes, “organizations are increasingly augmenting their existing email security solutions with additional layers of protection to more effectively mitigate targeted phishing attacks, such as business email compromise (BEC).” But the most effective augmentation is not another perimeter tool built on the same inspection model. It is a layer that operates and detects differently than SEGs, using behavioral context to identify malicious intent rather than known indicators and defined threat techniques.

Layering in Abnormal’s behavioral AI detection complements your existing Microsoft 365 investment, reduces risk and operational burden, and preserves a simpler architecture through API-based integration with Microsoft 365 instead of MX record changes.

The detection logic is different at each layer. Microsoft handles foundational protection for spam, malware, and other known threat techniques. Abnormal builds behavioral baselines across employees, vendors, devices, and communication patterns to identify anomalies that suggest manipulation or compromise. On average, the typical Abnormal customer sees 462 advanced attacks per month bypassing Microsoft native controls per 1,000 mailboxes.

That is what makes the two layers complementary rather than redundant. Microsoft covers a wide array of threats leveraging intelligence, signatures and payload analysis. Abnormal covers the attacks that require behavioral context, including text-only social engineering, compromised-vendor invoice fraud, suspicious internal mail, and account takeovers that do not present the classic red flags legacy tools depend on.

In other words, this is what defense in depth is supposed to look like: independent layers that cover different threat categories, not parallel tools inspecting the same signals in the same way.

The pattern is consistent once organizations move off the SEG: stronger protection against advanced attacks, less manual work for security teams, and better return on the native email platform they already have. More than 800 organizations have migrated over 3.5 million mailboxes from third-party SEGs to the Abnormal + native email model, and 76% of Abnormal customers now operate without a third-party SEG. Security leaders who have made the shift consistently report stronger coverage for advanced attacks, fewer false positives, and significantly less manual triage.

Peter Mueller of Saskatoon Public Schools put the economic logic plainly: “We didn’t want to pay for a secure email gateway to do the same security functions that Microsoft would do. If we were going to pay for email security, we wanted to get more from the solution.”

The results show what that looked like in practice. Within the first 180 days after replacing their third-party SEG with Abnormal, Saskatoon detected and auto-remediated more than 25,000 attacks, including 163 employee email account takeover attacks.

Boohoo saw the same pattern from the operational side, that shift meant catching advanced attacks the SEG had missed while saving the team up to 40 hours per month on manual email tasks.

Defense in depth is not about how many logos appear in the stack. It is about whether each layer covers risks the others miss.

If your SEG and your native controls are still relying on the same inspection model, you may have added cost and complexity without adding truly independent protection. But if Microsoft provides the foundational layer and Abnormal extends it with behavioral AI, the result is simpler architecture, better use of the Microsoft investment already in place, and stronger protection against the attacks that matter most.

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