6 Best email security solutions for Office 365 in 2026

Microsoft 365 secures more enterprise email than any other platform on the planet. Attackers know it, and they build their campaigns around it. Business email compromise, vendor impersonation, QR phishing, and AI-generated social engineering now routinely land in inboxes despite Microsoft's built-in defenses. Security teams that relied on Microsoft Defender alone are discovering gaps they cannot close fast enough.

This guide compares the leading email security solutions for Office 365 environments in 2026. For each vendor, we cover protection capabilities, deployment approach, visibility and control, automation, customer feedback, and overall fit. Whether you're adding a layer on top of Microsoft Defender, replacing a legacy gateway, or evaluating your first dedicated email security platform, this guide gives you the framework to make a confident decision.

What to know

• Microsoft 365's built-in defenses handle commodity threats well but leave consistent gaps in business email compromise, vendor impersonation, and novel phishing.
• The most important differentiator between platforms in 2026 is not just detection accuracy but how fast each platform adapts to new threats and how much analyst time the automation actually saves.
• API-native platforms deploy alongside Microsoft 365 without MX record changes, making them lower-risk to evaluate than legacy secure email gateways. See our full guide to Microsoft 365 email security for a deeper look at the architecture.
• Detection transparency matters: platforms that show you exactly why an email was flagged let analysts investigate faster, tune coverage without vendor tickets, and build confidence in every automated decision.
• The best way to compare solutions is a proof-of-concept run against your live mail flow. What vendors catch in your environment is more informative than any benchmark.

Why organizations add email security beyond Microsoft 365

Microsoft Defender for Office 365 provides a solid baseline. SafeLinks rewrites URLs, SafeAttachments sandboxes files, and anti-phishing policies add behavioral controls. For organizations already on Microsoft 365 E3 or E5, Defender is included and covers a meaningful portion of the threat landscape. The gaps show up in specific categories.

Sophisticated inbound threats

Business email compromise, thread hijacking, and vendor impersonation attacks rarely carry malicious links or attachments. They rely on context, trust, and social engineering. Microsoft Defender's detection model was optimized for malware and bulk phishing, not for zero-payload attacks that look like normal business correspondence. Security teams that run a passive evaluation alongside Defender routinely surface threats that Defender classified as clean.

Detection transparency and analyst control

When Microsoft Defender flags or misses a message, analysts get a spam confidence level and a policy verdict. There is no logic to inspect, no way to backtest a proposed adjustment, and no path to deploying a fix without navigating Microsoft's policy interface. Teams dealing with a new attack pattern targeting their specific industry, supplier base, or VIP list have limited options: adjust thresholds globally and accept more false positives, or wait for Microsoft to update its models.

Adaptation speed

Email attacks evolve faster than centralized detection models update. When a novel campaign starts hitting your environment, the window between first sighting and broad vendor coverage is measured in days or weeks, not hours. Organizations that need to close that gap faster are building their own detection layers on top of Microsoft's foundation.

Coverage of outbound and internal email

Microsoft Defender's primary focus is inbound threats. Outbound data loss prevention and internal email scanning for lateral movement or compromised accounts require Microsoft Purview, which adds cost and complexity. Dedicated email security platforms increasingly cover all three directions through a single policy engine, without routing mail to separate products.

Operational overhead

Every false positive creates work. Every missed threat creates an incident. Organizations that experience high false positive rates, slow abuse mailbox triage, or vendor-dependent tuning cycles eventually reach a point where the operational cost of managing the email security stack outweighs the licensing savings of relying solely on what Microsoft provides.

How we evaluated the best email security solutions for Office 365

The vendors in this guide were selected based on their relevance to Microsoft 365 environments, their market presence in 2026, and their representation in security team evaluations. The criteria below map to the questions security teams consistently raise before shortlisting a platform. We assessed each platform across the following:

• Detection coverage: How well does the platform detect BEC, novel phishing, vendor compromise, QR-based phishing, and zero-payload social engineering?
• Microsoft 365 integration: Does the platform connect natively to Microsoft 365 via API? What additional configuration is required?
• Deployment flexibility: Does the vendor support API-only, inline, or self-hosted deployments for organizations with data residency or sovereignty requirements?
• Detection transparency: Can analysts see the logic behind a verdict, inspect contributing signals, and adjust or override coverage without opening a vendor ticket?
• Automation: How much analyst time does the platform save on abuse mailbox triage, retroactive hunting, and detection updates?
• Adaptation speed: When a new attack pattern appears, how quickly does the platform generate new coverage?
• Customer feedback: What are verified reviewers on G2 and Gartner Peer Insights saying about detection accuracy, false positive rates, ease of use, and support?
• Operational fit: Which types of organizations and security team structures does this platform serve best?

Detailed reviews of the best email security solutions for Office 365

1. Sublime Security

Sublime is an agentic email security platform built for Microsoft 365 and Google Workspace environments. Every verdict traces back to readable detection logic that analysts can inspect, edit, backtest against 30 days of historical mail, and deploy without filing a vendor support ticket. Two agents run the operational work: ASA (Autonomous Security Analyst) reviews reported emails in seconds, while ADÉ (Autonomous Detection Engineer) writes and deploys new org-specific detection logic your team can see, adjust, test against past mail, and turn on right away, no vendor ticket required.

That transparency and speed come from the underlying architecture: rather than applying a single shared model across all customers, Sublime builds coverage tailored to each organization's specific threat patterns, supplier relationships, and communication norms. It connects via API, covering email across all directions through a single detection engine and policy layer.

Key strengths:

• Transparent detection logic that analysts can read, edit, and deploy without vendor involvement
• ASA reviews reported emails from one place, cutting response time from hours to seconds.
• ADÉ generates new org-specific detections in hours, validated against your actual historical mail before anything goes live
• Deploys as multi-tenant SaaS, single-tenant SaaS, or fully self-hosted (including AWS GovCloud and FedRAMP environments)
• Covers every direction email moves: inbound threats, internal traffic, and outbound data loss prevention (in beta).
• Backtesting lets teams validate new detection logic against real historical mail before deploying

Customer feedback: Sublime holds a 4.9/5 rating on G2 from verified enterprise reviewers, with 14 Enterprise badges in G2's Spring 2026 reports. Gartner Peer Insights reviewers highlight detection accuracy, ease of setup, and the transparency of detection logic as primary strengths. Sublime won Best Security Solution for Email in the 2026 Security Awards and was named to Fast Company's 2026 World's Most Innovative Companies list.

Best for: Security teams at mid-market and enterprise organizations that want high detection efficacy, full transparency into every automated decision, and a platform that adapts to their specific environment without vendor bottlenecks. Particularly strong for organizations with data residency requirements, those augmenting or replacing a legacy gateway, and teams running Microsoft 365 without additional email security today.

2. Abnormal AI

Abnormal AI is one of the most widely deployed behavioral AI platforms in enterprise email security. It deploys via API and leads with a low-admin, easy-deployment narrative.

Core approach: Abnormal uses a behavioral AI model to establish per-organization baselines and flag deviations. Its Attune 1.0 model learns from patterns across Abnormal's whole customer base.

Strengths: Fast deployment, strong brand recognition, wide Fortune 500 adoption, a polished demo, and ready-made connections to popular security tools like Splunk and Microsoft Sentinel.

Common tradeoffs to evaluate: Detection 360, Abnormal's transparency feature, shows behavioral reasoning behind verdicts in a read-only view. Analysts can request new detection logic in plain language without filing a ticket, but they still can't see or edit the underlying logic itself, unlike a platform built for hands-on tuning. When a new attack pattern appears or a detection misses, the adaptation path runs through Abnormal's release cycle rather than your security team. Organizations checking their Borderline mail folder often find a lot of legitimate email caught there, worth testing directly in a trial. Separately, reviewers report high false-positive rates specifically on Abnormal's account-takeover alerts.

AISM (abuse mailbox automation) is a separately priced add-on, as are ATO protection, graymail filtering, and Security Posture Management, meaning the all-in cost is materially higher than the base price.

G2 rating: 4.8/5 (71 reviews)

Best for: Teams that want a fast-to-deploy AI layer and are fine with the vendor controlling detection updates. Abnormal is cloud-only, though it does carry a federal security certification, so some public-sector buyers can still consider it.

For teams evaluating Abnormal, see our best Abnormal Security alternatives guide for a deeper comparison.

3. Proofpoint

Proofpoint is the most widely deployed enterprise secure email gateway, installed at 85% of Fortune 100 companies according to the company. Its primary product is Email Protection (a legacy SEG architecture), augmented by Tessian as an API layer for internal and BEC-class threats.

Core approach: Proofpoint operates a Centralized Detection Model where vendor-managed detection logic applies uniformly across all customers. Proofpoint has said twice now, in March and again in June 2026, that its two products now work as one. In practice, it's still two separate consoles and two separate detection systems stitched together.

Strengths: Broad platform coverage (SEG, DLP, SAT, archiving), established enterprise relationships, 2025 Gartner Magic Quadrant Leader (highest in Ability to Execute), and the recent Hornetsecurity acquisition extending reach into mid-market and MSP channels.

Common tradeoffs to evaluate: Detection transparency is limited. Proofpoint returns spam scores rather than inspectable detection logic, and tuning requires support tickets. Organizations migrating from on-premises Exchange to Microsoft 365 often find that the gateway architecture creates complexity that the API-native alternatives do not. Proofpoint's Satori AI agents were announced in 2026 but were still rolling out, not yet widely available, as of April.

G2 rating: ~4.6/5 (584 reviews)

Best for: Large enterprises with deeply integrated Proofpoint infrastructure, organizations that need a single vendor covering email security, DLP, and compliance archiving, and teams currently running a legacy SEG that are not yet ready to change mail routing.

For a direct comparison, see our best Proofpoint alternatives guide.

4. Mimecast

Mimecast is a gateway-centric platform covering email security, archiving, continuity, DMARC, brand protection, and web security, and has recently expanded into employee security-awareness training. It deploys as a Cloud Gateway (SEG) or Cloud Integrated (API) configuration.

Core approach: Mimecast's detection relies on CyberGraph AI and its impersonation, URL, and attachment protections, and it's also building out connections to other security tools like CrowdStrike.

Strengths: Strong archive and continuity capabilities (M2A), DMARC management, broad module coverage, and a long-standing customer base in regulated industries. Mimecast is frequently kept in place for archive and continuity while the email detection layer is replaced or augmented.

Common tradeoffs to evaluate: Detection transparency is limited, with mentions of difficulty understanding why specific emails were flagged during evaluations. Investigation workflows across modules can require context-switching between consoles. In head-to-head testing, some customers have found malicious messages that Mimecast missed.

G2 rating: ~4.4/5 (320 reviews)

Best for: Organizations with compliance-driven archiving and continuity requirements that want a single vendor. Common pattern: Mimecast is retained for archive and continuity while Sublime handles detection and investigation.

For a direct comparison, see our Mimecast alternatives guide.

5. Microsoft Defender for Office 365

Microsoft Defender for Office 365 is included at no additional cost in Microsoft 365 E3 and E5 subscriptions (at different capability tiers). It covers inbound threats to email and collaboration tools including Teams, SharePoint, and OneDrive, with features including SafeLinks, SafeAttachments, anti-phishing policies, and Attack Simulation Training.

Core approach: Microsoft Defender is a native Microsoft 365 integration, managed through the Microsoft 365 Defender portal. It uses Microsoft's global threat intelligence and telemetry across the Microsoft ecosystem, which provides broad coverage for commodity threats and malware.

Strengths: No additional cost for E3/E5 subscribers, native integration across the Microsoft stack, real-time threat intelligence from Microsoft's global visibility, and strong coverage for commodity phishing and malware.

Common tradeoffs to evaluate: Detection gaps appear most clearly in zero-payload attacks: BEC, vendor impersonation, and thread hijacking where there is no malicious link or attachment to scan. Analyst control over detection logic is limited compared to dedicated platforms. False positive management and tuning require navigating Microsoft's policy interface, and coverage adaptation depends on Microsoft's centralized update cycle. Gartner Peer Insights reviewers note the interface as less intuitive than dedicated platforms, and detection gaps in sophisticated attack categories are a recurring theme.

G2 rating: ~4.5/5

Best for: Organizations that want a baseline inbound threat layer already included in their Microsoft 365 subscription. Most organizations running E3 or E5 should use Defender as a foundation and evaluate whether a dedicated platform is needed to close specific detection gaps.

6. IRONSCALES

IRONSCALES is a cloud-native email security platform positioning around mid-market and MSP use cases. It combines AI-driven detection, autonomous remediation, and built-in phishing simulation and security awareness training in a single platform.

Core approach: IRONSCALES uses a combination of AI detection and crowdsourced threat intelligence from its customer base. The platform includes a built-in security awareness training module, positioning it as an all-in-one option for organizations that want email security and user training from a single vendor.

Strengths: Built-in SAT integration removes the need for a separate training platform, fast setup typically completed same-day, strong customer satisfaction ratings on G2, and a clear mid-market fit.

Common tradeoffs to evaluate: IRONSCALES does not publish its detection logic or offer custom detection authoring in the way dedicated security platforms do. Before evaluating, ask the vendor how detection logic is structured, whether analysts can inspect or modify individual verdicts without opening a support ticket, and how the platform handles data residency requirements for enterprise deployments.

G2 rating: ~4.7/5 (65 reviews)

Best for: Mid-market organizations and MSPs that want a single platform covering email security and user security awareness training, and teams that prioritize simplicity of operation over deep detection customization.

How to choose the right email security solution for Office 365

Start with your current Microsoft 365 plan

If you're on Microsoft 365 E5, Defender for Office 365 Plan 2 is already included. Run a baseline assessment of what it catches before adding anything else. Most organizations that do find gaps, and those gaps define what you need from an additional layer.

Define what detection transparency means for your team

Every platform in this guide runs without requiring manual configuration from day one. The difference is what teams can do when they want more. Some security teams want full visibility into why every email was flagged and the ability to write, test, and deploy their own detections when something new emerges. Others are happy to let the platform run and never open the logic at all. Transparent, customizable detection logic tends to matter most to enterprise and mid-market security teams with dedicated SOC analysts. For lean teams managing broad environments with limited headcount, the hands-off automation story is the bigger priority.

Evaluate API-native vs. gateway architecture

For email security for Microsoft 365 specifically, using an API-native deployment is generally the right architecture. API-native platforms connect directly to Microsoft 365 via Microsoft's Graph API, require no MX record changes, and can be deployed passively alongside your current setup for evaluation. Gateway-based platforms require routing mail through the vendor's infrastructure, which adds complexity and creates a dependency on the vendor's uptime.

See our guide to API-based email security vs traditional SEG for a detailed breakdown of the architectural tradeoffs.

Assess automation depth

Abuse mailbox triage is often the highest-volume operational task for email security teams. Ask each vendor: does your platform automate triage of both user-reported and system-flagged messages, or only user-reported? Is that automation included in the base price or separately licensed? What does the remediation look like when ASA or an equivalent agent acts on a message? For teams that also need proactive coverage, ask how the platform handles behavioral threat hunting against historical mail.

Consider your threat profile

Email security for financial services environments, healthcare organizations, and critical infrastructure face different attack patterns than software companies or retailers. Platforms with org-specific detection coverage that adapts to your supplier relationships, communication norms, and industry-specific attack patterns will outperform generic shared models in these environments.

Validate with a proof of concept

Every vendor in this guide should be willing to run a passive evaluation alongside your current stack. The POC is the most reliable evaluation method because it tests detection performance against your actual mail flow, not a synthetic benchmark. When you run a POC, define success criteria before it starts: threats detected above your current baseline, false positive rate, analyst hours saved, and time-to-new-coverage when a new attack pattern is introduced.

Why organizations choose Sublime Security for Microsoft 365

Security teams choose Sublime when detection efficacy and analyst control matter more than vendor lock-in and bundled scope.

The organizations that fit Sublime best tend to share a few characteristics. They're running Microsoft 365 and have found that Defender covers commodity threats but leaves gaps in the specific attack categories they care most about. They want to see exactly why an email was flagged, not because they distrust automation but because transparency is what makes automation trustworthy. And they want coverage that closes gaps in hours, not one that sends them back to a vendor ticket queue when something new shows up.

Sublime's Distributed Detection Model means coverage is built around your organization's specific threat patterns from day one, starting with the Core Feed and expanding continuously as ADÉ generates new detections tailored to what it sees in your environment. Full protection starts on day one, and coverage keeps expanding to match your organization from there.

For organizations with data residency or sovereignty requirements, Sublime's deployment flexibility is a structural differentiator. Self-hosted options are available on AWS GovCloud, plus single-tenant cloud hosting for teams that need extra data isolation without going fully self-hosted.

AI email security built around autonomous agents is different from AI email security built around a single shared model. For teams tired of filing tickets to close a coverage gap, that difference is the whole evaluation.

FAQs about the best email security for Office 365

How do I know if my organization needs additional email security beyond Microsoft 365?

The clearest signal is a gap in the attack categories covered in the "Why organizations add email security beyond Microsoft 365" section above: zero-payload threats like BEC, vendor impersonation, and thread hijacking that Microsoft Defender was not built to catch. If your team is regularly investigating emails that bypassed Defender, or spending significant analyst time on abuse mailbox triage, those are strong indicators. The fastest way to find out is to run a passive API-native evaluation alongside your current Defender deployment. Most organizations that do this surface threats they didn't know were reaching inboxes.

What are the benefits of adding a third-party email security solution to Office 365?

The primary benefits are detection coverage for the attack categories Defender misses, faster adaptation when new attack patterns emerge, and operational automation that reduces analyst workload. Depending on the platform, you also get detection transparency (the ability to see and control the logic behind every verdict), org-specific coverage tuned to your environment, and deployment flexibility for organizations with compliance or data residency requirements.

How long does it take to deploy an email security solution in a Microsoft 365 environment?

For API-native platforms, deployment is typically measured in hours, not days. The platform connects directly to Microsoft 365 via API, and most organizations are ingesting live mail flow and generating initial detection coverage within a day of starting the setup process. Gateway-based platforms require routing changes and typically take longer to deploy and validate. For a broader readiness checklist, see our guide to email security best practices.

What should organizations look for when comparing Office 365 email security solutions?

Five areas matter most: detection coverage for zero-payload attacks, detection transparency and analyst control over the logic behind verdicts, automation depth for abuse mailbox triage and coverage updates, adaptation speed when new threats emerge, and deployment flexibility for organizations with data residency or sovereignty requirements. Price comparisons should account for add-on licensing, since several vendors in this category offer a low base price and charge separately for core capabilities like abuse mailbox automation.

Why do organizations choose Sublime Security over other Office 365 email security solutions?

The most common reason, across documented wins, is the combination of detection efficacy in a POC and the ability to see and control the detection logic that drives automated decisions. Organizations that have run passive evaluations alongside Abnormal AI, Proofpoint, and Microsoft Defender consistently surface threats those platforms classified as clean. The transparency of Sublime's detection logic, and the fact that analysts can adjust and deploy coverage without vendor involvement, is a differentiator for security teams that need to respond to new attack patterns faster than a vendor update cycle allows.

What types of organizations are the best fit for Sublime Security?

Sublime serves mid-market and enterprise organizations across industries. The strongest fits share a few characteristics: they're running Microsoft 365 (or Google Workspace) and need detection coverage beyond what the native platform provides; they value transparency and analyst control over automated decisions; or they have data residency, sovereignty, or deployment flexibility requirements that eliminate SaaS-only vendors. Organizations in financial services, technology, media, healthcare, and defense have been among the most active adopters. For a broader comparison of the market, see our guide to the top email security companies in 2026.

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