How effective is Microsoft Defender against phishing attacks?

Phishing emails remain the most common and costly cyber threat targeting small and mid-sized businesses, yet many rely on Microsoft Defender alone to stop them.

That’s a big problem because attackers continue to outpace basic defenses with techniques like business email compromise (BEC), brand impersonation, and AI-generated phishing emails.

Making matters worse, attackers are overwhelmingly focused on Microsoft 365 as a single, high-value target. Because M365 dominates the enterprise productivity market, threat actors concentrate their efforts on exploiting its users through domain spoofing, M365 account compromise, OAuth token theft, and malware distributed via M365 collaboration tools like Teams, OneDrive, and SharePoint. Phishing-as-a-service platforms such as Tycoon2FA are purpose-built to target M365 environments, and Microsoft itself reported blocking more than 13 million Tycoon2FA-linked malicious emails in a single month in late 2025. This level of dedicated, M365-specific attacker focus means that businesses relying solely on native defenses are defending against an adversary who knows those defenses inside and out.

So how do you respond? Leading organizations strengthen Microsoft Defender with Mimecast’s advanced phishing protection to close critical gaps, reduce false positives, and achieve a defense-in-depth strategy that works better together.

In this article, you’ll learn how Microsoft Defender detects phishing, where it falls short, and how pairing it with Mimecast delivers stronger protection for your business.

How does Microsoft Defender detect phishing emails?

Microsoft Defender detects phishing emails using a layered approach that combines machine learning, artificial intelligence, and threat intelligence. It verifies sender identity through SPF, DKIM, and DMARC, analyzes domains with global telemetry, and scans links and attachments with Safe Links and Safe Attachments. Defender also monitors communication patterns to identify impersonation attempts and advanced phishing campaigns.

Despite these layers, 37% of respondents in our State of Email and Collaboration Security Report said Microsoft 365’s native security failed to block malware without additional tools, and three‑quarters of companies have already implemented or are rolling out DMARC to combat spoofing.

While Microsoft Defender provides a strong baseline of phishing protection, its effectiveness often depends on the type of attack. Payload-based phishing is usually blocked, but more subtle social engineering tactics like business email compromise are harder to detect because they don’t rely on malicious links or attachments.

For SMB security teams, this creates a challenge: Defender can stop many phishing attempts, but blind spots remain where human trust is exploited instead of malware. These gaps mean attackers may still reach end users, increasing the risk of costly breaches.

The complexity of getting Microsoft Defender to perform at its best is a challenge many organizations can't solve on their own. According to Forrester, AI adoption, vendor consolidation, and Microsoft's continued investment in a unified security platform contributed to a 20% increase in the expected partner revenue opportunity at enterprise customers over the past 15 months—meaning companies are projected to spend significantly on third-party services just to implement and optimize Defender in 2026. For SMBs with fewer resources, this underscores why a solution that works effectively out of the box, rather than one that demands heavy configuration and consulting spend, is critical.

By combining Microsoft Defender with Mimecast’s Advanced Email Security, organizations gain layered protection that stops both common phishing campaigns and advanced, targeted threats. This partnership helps SMBs stay protected against both known malicious payloads and stealthier impersonation attempts.

Critically, Mimecast doesn't just sit alongside Microsoft 365—it makes M365 smarter. Through API-based integrations, Mimecast shares threat intelligence directly with Microsoft Defender, automatically pushing malicious file hashes, suspicious URLs, and dangerous domains detected by Mimecast into Defender's own detection rules. In return, endpoint compromise signals from Microsoft Defender flow back into Mimecast, enriching email security detection. This bidirectional intelligence sharing means every threat blocked by Mimecast strengthens your M365 environment's defenses in real time, creating a continuously improving security loop rather than two isolated tools.

Related: Does Microsoft Defender replace the need for email security gateways? → 

What types of phishing does Microsoft Defender block?

Microsoft Defender blocks a wide range of phishing types, including bulk phishing, spear phishing, and whaling. It stops malicious links and attachments with Safe Links and Safe Attachments, protects against credential theft and account takeover, and uses Enhanced Phishing Protection to prevent password entry on malicious or spoofed websites.

That’s important when 76% of organizations are bracing for the fallout of an email‑borne attack and 97% report experiencing at least one phishing incident in the past year.

This coverage gives smaller businesses relief from the most obvious phishing scams, cutting down on noisy alerts and lowering the chance of employees clicking on generic, mass-distributed emails.

The limitation is that Microsoft Defender’s strength lies in detecting technical indicators like links, attachments, or known domains. Attacks that rely on social engineering, such as business email compromise or QR code phishing, often bypass these controls because they don’t include a detectable payload.

Mimecast’s solutions complement Microsoft Defender by closing the gaps attackers exploit (like impersonation and BEC), stopping phishing attempts before they reach employees. Together, the two solutions give coverage against both mass phishing campaigns and precision attacks aimed at executives or finance teams.

Related: Can Microsoft Defender stop business email compromise? → 

How quickly does Microsoft Defender adapt to new phishing techniques?

Microsoft Defender adapts to new phishing techniques by updating its machine learning models, Safe Links, and Safe Attachments scanning engines with threat intelligence from across Microsoft 365 environments. While these updates are frequent, attackers often innovate faster, creating periods where emerging techniques bypass detection.

Examples of emerging phishing techniques that challenge Microsoft Defender include:

• QR code phishing (quishing) that hides malicious URLs in scannable codes.
• Image-based phishing where text or links are embedded in graphics to bypass filters.
• AI-generated phishing emails that mimic tone, grammar, and branding more convincingly than traditional scams.
• Multi-channel phishing that moves beyond email into Teams, OneDrive, or other collaboration platforms.

Additionally, attackers are deploying M365-specific techniques like OAuth device code phishing, where victims are tricked into authenticating on legitimate Microsoft login pages, handing over tokens that bypass MFA entirely. These campaigns, observed targeting the tech, manufacturing, and financial sectors since late 2025, grant attackers persistent access to email, files, and admin functions—all without stealing a single password.

Surveyed security leaders share this concern: 80% worry about AI‑spawned attacks, and 67% concede that such attacks are inevitable in the coming months.

Because Defender’s defenses are updated reactively, attackers can exploit gaps before patches or model updates take effect. SMBs often lack the resources to identify and respond quickly enough, leaving employees vulnerable during this adaptation lag.

Adding Mimecast to Microsoft Defender helps organizations reduce the risk of BEC, false positives, and emerging phishing attacks that native tools often miss. Mimecast’s AI-powered analysis adapts in real time, making certain SMBs have faster protection against the latest phishing innovations.

Can Microsoft Defender protect collaboration tools like Teams and OneDrive?

Microsoft Defender provides limited protection for collaboration tools like Teams and OneDrive by scanning shared files and links for known threats. While this helps block some malware and phishing attempts, it does not deliver comprehensive visibility or advanced detection across the broader collaboration ecosystem.

This is especially concerning given that attackers increasingly treat the entire M365 ecosystem—not just email—as a single attack surface. Malware originating from compromised M365 accounts can spread laterally through Teams chats, SharePoint sites, and OneDrive shared files, exploiting the inherent trust users place in content shared by colleagues within the same platform.

Our data underscores why this gap matters: seven in ten respondents say collaboration tools pose urgent new threats and almost as many believe a collaboration‑tool‑based attack on their organization is likely or inevitable.

For SMBs, this gap is significant. Employees increasingly share sensitive information through Teams, OneDrive, and other Microsoft 365 apps, making these platforms attractive targets for attackers. Native protections focus mainly on file scanning, leaving more subtle phishing and impersonation attacks undetected.

Collaboration Security: Microsoft Defender vs. Mimecast

Moreover, 59% of employees regularly use unvetted collaboration apps and 61% of security leaders feel the native protections in these tools are inadequate, while nearly half of organizations have already deployed extra layers of software and 47% provide tool‑specific security awareness training.

Because phishing has moved beyond the inbox, SMBs that rely only on Defender for collaboration protection may underestimate their exposure. Attackers exploit these blind spots to bypass traditional email defenses and reach employees where they work most.

Together, Microsoft Defender and Mimecast provide the resilience SMBs need to block advanced phishing while keeping business communication flowing without disruption. Mimecast extends protection into Teams, SharePoint, and OneDrive, helping stop attackers from exploiting blind spots in everyday collaboration.

What phishing attacks bypass Microsoft Defender?

Phishing attacks bypass Microsoft Defender through advanced tactics like business email compromise, adversary-in-the-middle (AiTM) token theft, and social engineering that impersonates trusted brands. Attackers also exploit zero-day vulnerabilities and filtering gaps, such as Advanced Delivery policies, to evade payload-based defenses focused on links, attachments, and known malicious domains.

Recent research highlights just how laser-focused these attackers have become on M365 environments. Threat actors are exploiting M365's Direct Send feature—designed for internal devices like printers—to send spoofed phishing emails that appear to come from within the organization, bypassing both Microsoft's own filtering and third-party email security solutions. These campaigns impersonate HR departments, IT security teams, and executives, and have been observed at scale across multiple industries since mid-2025. When the platform you rely on for daily work is the same platform attackers specialize in exploiting, single-vendor security is no longer enough.

Our Human Risk 2025 report links these gaps to user behavior: 95% of data breaches are attributed to human error, 8% of employees account for 80% of incidents, and 43% of organizations saw an increase in internal threats or data leaks in the past year, with insider‑driven data‑exposure events costing an average of US $13.9 million.

Because these campaigns avoid obvious indicators like malicious links or attachments, Defender’s payload-based protections often cannot detect them. As a result, SMBs face elevated risk of financial fraud, account takeover, and reputational damage.

Mimecast extends Microsoft Defender by surfacing high-risk users and stopping phishing techniques designed to exploit human trust rather than technical vulnerabilities. This added visibility makes it possible to better protect executives, finance staff, and other high-value targets.

Related: Is Microsoft Defender enough for small business security? → 

How accurate is Microsoft Defender in detecting phishing without false positives?

Microsoft Defender provides strong phishing detection but is not flawless. It can generate false positives that delay communication and false negatives that miss advanced threats. Accuracy varies across Defender products, and Microsoft is enhancing AI-powered features like its Phishing Triage Agent to reduce misclassifications and improve reliability.

Key drivers of misclassification in Microsoft Defender include:

• Authentication gaps where legitimate senders lack properly configured SPF, DKIM, or DMARC.
• Heuristic errors when unusual but safe communication patterns trigger filtering rules.
• Inconsistent performance across Defender products, leading to uneven protection.

Independent testing by SE Labs in 2024 found that while Defender blocked all known malware and phishing, it achieved only 85% overall accuracy due to false positives and missed social engineering threats.

These issues leave small businesses struggling to balance security with business productivity. When email flow is disrupted or phishing threats are missed, the result is wasted time, higher risk, and frustrated employees.

Pairing Microsoft Defender with Mimecast allows SMBs to simplify security operations while improving protection against sophisticated phishing tactics. Mimecast’s granular policy controls and AI-driven detection help security teams cut through noise and focus on real threats instead of wasted investigations.

Does Microsoft Defender provide enough visibility into targeted users?

Microsoft Defender provides visibility into phishing trends but limited user-level insights in its basic form. Enterprise versions, particularly Microsoft Defender XDR, expand visibility across endpoints, identities, and email, enabling security teams to pinpoint which employees are most frequently targeted and deliver more focused protection against advanced phishing campaigns.

The visibility gap is particularly challenging for SMBs using Business or E3 plans. Without user-level insights, they struggle to see who’s most at risk and to focus their training where it matters most.

Key visibility gaps for smaller organizations include:

• Limited reporting that highlights overall phishing activity but not individual user patterns.
• No risk scoring to identify which employees are most susceptible.
• Reactive alerts that flag blocked attacks but provide little predictive insight.

These blind spots make it harder for SMBs to stop repeated targeting of executives, finance teams, or employees with privileged access—groups most likely to be hit with BEC or spear-phishing campaigns.

Microsoft Defender delivers strong baseline coverage, but Mimecast strengthens it with advanced detection and user-level visibility—making the two more effective when used together. This deeper visibility helps SMBs prioritize interventions for the employees most likely to be targeted.

Mimecast's API integration with Microsoft 365 also enhances this visibility. By ingesting phishing detection events from Microsoft Defender for Office 365 into Mimecast's Human Risk Command Center, security teams gain a unified view that correlates user behavior patterns with actual phishing attacks—turning fragmented signals from both platforms into actionable, user-level risk scores that neither tool could produce alone.

How much configuration and expertise does Defender require for phishing protection?

Microsoft Defender requires significant configuration and ongoing expertise to deliver optimal phishing protection. Security teams must tune multiple policies, manage authentication settings, and regularly update rules. Without specialized knowledge, SMBs risk leaving default settings in place, which can create gaps attackers exploit.

This configuration burden has real financial consequences. The Forrester data showing a 20% increase in the expected partner revenue opportunity for enterprise customers confirms that organizations are increasingly turning to—and paying for—outside help to get Defender configured correctly. For SMBs that can't justify enterprise-level consulting engagements, this complexity gap becomes a security gap.

For smaller organizations, this complexity can be overwhelming. Defender’s security capabilities are powerful but fragmented across different consoles and policy sets. This means protection quality often depends less on the technology itself and more on the skills and bandwidth of the IT team.

Configuration and Expertise: Microsoft Defender vs. Mimecast

When misconfigurations occur, the result can be false positives that slow business or gaps that allow sophisticated phishing attacks through. This places SMBs in a difficult position—overly restrictive settings frustrate users, while overly permissive settings expose the organization.

Forward-looking SMBs enhance Microsoft Defender with Mimecast’s AI-driven protection to gain the same level of phishing defense trusted by leading enterprises. With simplified deployment and fewer manual configurations, SMBs can get enterprise-grade protection without expanding their IT team.

How does Microsoft Defender compare to Mimecast for phishing protection?

Microsoft Defender provides strong baseline phishing protection as part of Microsoft 365, while Mimecast delivers advanced, AI-powered defenses designed to stop targeted attacks like business email compromise, impersonation, and quishing. Together, they offer SMBs a layered security strategy that balances cost, coverage, and operational efficiency.

For SMBs, the key difference is depth. Defender focuses on blocking known payloads and large-scale phishing, but it struggles with sophisticated social engineering and emerging tactics. Mimecast adds behavioral analysis, impersonation detection, and human risk insights to stop the advanced threats that cause the greatest damage.

Microsoft Defender vs. Mimecast for Phishing Protection

For SMBs, the “why” is clear: relying on Defender alone leaves exposure to the most costly phishing attacks. Mimecast closes these gaps while reducing administrative burden, helping smaller teams focus on business instead of constant fine-tuning.

Leading organizations combine Microsoft Defender with Mimecast’s Advanced Email Security and Human Risk Management to achieve defense-in-depth protection. This better-together approach ensures that both the common and the sophisticated phishing threats are stopped before they reach employees.

Conclusion: Microsoft Defender + Mimecast = Complete Coverage

Microsoft Defender offers a solid first line of phishing defense, but it was not designed to stop every advanced tactic attackers use today. Mimecast adds the AI-powered analysis, impersonation detection, and user-level visibility needed to close those gaps.

With attackers dedicating unprecedented focus to the M365 ecosystem—from domain spoofing and OAuth token theft to malware spreading through Teams and OneDrive—the case for layered defense has never been stronger. And as Forrester data confirms, even enterprise organizations are spending significantly more on third-party services to optimize Defender, reflecting the reality that native tools alone require substantial investment to reach their full potential. Mimecast's API-driven integration goes further than bolting on an extra layer: by sharing threat intelligence bidirectionally with Microsoft Defender, it makes the entire M365 security stack smarter with every threat it detects.

Together, Microsoft and Mimecast give SMBs the comprehensive phishing protection required to safeguard employees, data, and customer trust.

Ready to strengthen your phishing defenses? See how Mimecast works alongside Microsoft Defender to protect your business from advanced threats. Get your free M365 Threat Scan today →