GreatHorn
Cloud email security vendor, fully absorbed into Proofpoint since 2022, no longer independent.
The call
GreatHorn no longer exists as a company; its code lives inside Proofpoint, which is now the one fighting Microsoft Defender and Abnormal Security for the email security market.
GreatHorn was an email security company that Proofpoint bought and folded into its larger product in September 2022. The real question now is whether Proofpoint itself (owned by private equity firm Thoma Bravo since 2021) will stay strong over the next few years. Microsoft's bundled email protection in Microsoft 365 is winning over mid-sized companies with a good-enough product at a low price, and newer AI models like Claude are making phishing attacks more convincing, which should help email security companies but makes GreatHorn's older detection method less special. The picture could flip if Proofpoint can roll out new AI-powered features by mid-2026 that actually work better than what Microsoft offers and if companies keep paying premium prices for security separate from their Microsoft bundle.
Why
01
AI pressure: Anthropic Claude and other frontier models used to craft sophisticated phishing. AI-written phishing gets harder to catch with old rules, pushing buyers toward AI-native tools Proofpoint is racing to ship. Could AI do the core job? Only partly.
02
Bigger players: Microsoft Defender for Office 365. Bundled free into M365, Defender is eating the mid-market and creeping into enterprise deals Proofpoint used to win outright. Could a giant copy it? Yes, easily.
03
What protects it. Proofpoint's 10,000+ large enterprise contracts and Fortune 100 penetration make ripping it out costly and slow.
What would change this call
Microsoft Defender for Office 365 adoption climbs further in Fortune 1000 mid-market accounts by 2027. Would show bundled Microsoft security squeezing Proofpoint's enterprise pricing power.
Abnormal Security pursues an IPO or discloses head-to-head wins against Proofpoint in enterprise deals. Would show a standalone AI-native rival displacing Proofpoint's email security position.
Thoma Bravo announces a sale, secondary, or IPO process for Proofpoint. Would signal a liquidity event resolving the PE-ownership uncertainty around Proofpoint.
Abnormal Security pursues an IPO or discloses head-to-head wins against Proofpoint in enterprise deals. Would show a standalone AI-native rival displacing Proofpoint's email security position.
Thoma Bravo announces a sale, secondary, or IPO process for Proofpoint. Would signal a liquidity event resolving the PE-ownership uncertainty around Proofpoint.
Key numbers
- Proofpoint holds ~43% email security market share
- GreatHorn brand down to ~0% share, ~12 customers
- Proofpoint bought for $12.3B by Thoma Bravo
Named risks
- Microsoft Defender for O365 bundling compresses Proofpoint's enterprise pricing power and mid-market demand.
- Frontier AI models like Claude increase phishing sophistication faster than defenses can adapt, per Proofpoint's own June 2026 commentary.
- Real-world cases show Proofpoint's URL-rewriting security branding exploited by attackers to disguise phishing links, per May 2026 IRONSCALES research.
- Thoma Bravo private equity ownership creates equity illiquidity risk for any evaluation of Proofpoint as a durable bet.
- GreatHorn brand itself is orphaned with near-zero independent traction, offering no standalone value to assess.
Rivals, as read in July 2026
| Rival | Direction | Evidence |
|---|---|---|
| Microsoft Defender for Office 365 | gaining on | Bundled into M365 licensing, eroding mid-market and enterprise email security demand as of 2026 |
| Abnormal Security | gaining on | Won head-to-head enterprise deal displacing Proofpoint at industrial manufacturer, per case study dated 2026-07-21 |
| Mimecast | contesting | Permira-owned rival competing in secure email gateway market, made acquisitions through 2024 |
Questions people ask
- Is GreatHorn still an independent company?
- No. GreatHorn was acquired by Proofpoint in September 2022 and no longer operates as a standalone entity. Its team, brand, and technology were fully absorbed into Proofpoint's threat-protection portfolio, with the former GreatHorn headcount shrinking from 23 in April 2022 to just 10 by 2024.
- Who owns GreatHorn now?
- GreatHorn's technology and team belong to Proofpoint, which acquired it in September 2022. Proofpoint itself is privately held by Thoma Bravo, which completed a $12.3 billion take-private deal on August 31, 2021, per Proofpoint's own press release.
- Could AI replace GreatHorn's technology?
- GreatHorn's original behavioral email-detection niche has become commodity. Its tech now lives inside Proofpoint's Adaptive Email Security, which uses deep learning, LLMs, and NLP across 250+ data points, documented as of August 2024, largely superseding GreatHorn's standalone approach.
- Who is GreatHorn's biggest competitor now?
- Since GreatHorn was absorbed into Proofpoint in 2022, the relevant competitive threats are Microsoft Defender for Office 365, bundled into M365 licensing and eating mid-market share, and Abnormal Security, which per a July 2026 case study won a head-to-head deal displacing Proofpoint at an industrial manufacturer.
- How much funding did GreatHorn raise before being acquired?
- GreatHorn raised roughly $22.18 million total per CB Insights, across a $2.25 to $2.53 million seed round (March 2016), a $6.3 million Series A (June 2017), and a $13 million Series A-1 (June 2019) co-led by RRE Ventures and .406 Ventures, before its September 2022 acquisition.
- Did GreatHorn's founder stay on after the Proofpoint acquisition?
- Yes. GreatHorn co-founder and CEO Kevin O'Brien transitioned to Proofpoint after the September 2022 acquisition and held the title of SVP and GM for Email Security Products as of a February 2024 report.
Timeline
- 2026-07-16Proofpoint launches Satori agentic abuse-mailbox agent and Core Email Protection API
- 2026-06-30Proofpoint targets SEG plus API unification into a single email protection model
- 2026-06-11Proofpoint cites frontier AI models escalating phishing sophistication in security blog
- 2026-03-23Proofpoint unveils agentic-workspace email and DSPM tools, claims 99.999% detection efficacy
- 2025-05-15Proofpoint announces acquisition of Hornetsecurity, including Vade, to expand email security scope
- 2022-09Proofpoint acquires GreatHorn, absorbing its team, brand, and IP
- 2021-08-31Thoma Bravo completes $12.3 billion take-private acquisition of Proofpoint
- 2019-06-26GreatHorn raises $13 million Series A-1 co-led by RRE Ventures and .406 Ventures
- 2017-06GreatHorn raises $6.3 million Series A led by .406 Ventures and Techstars Venture Capital Fund
- 2016-03-21GreatHorn secures $2.25 to $2.53 million seed round led by ff Venture Capital and SoftTech VC
Revision history
21 July 2026
First assessed. Protection under test. The text above is fixed as of this date and is not edited afterwards.
In the same band
Sources
- Where is GreatHorn Located? HQ, Global Offices & Company Insights (2024-02-06)
- Revenue, Competitors, Alternatives (2022-04-16)
- Adaptive Email Security
- Products, Competitors, Financials, Employees, Headquarters Locations (2024-10-04)
- GreatHorn nabs $13 million for cloud-hosted email security software (2019-06-26)
- GreatHorn raises $13 million for cloud-hosted email security software (2019-06-27)
- Thoma Bravo Completes Acquisition of Proofpoint (2021-08-31)
- Proofpoint Redefines Email and Data Security for the Agentic Workspace (2026-03-23)
- Market Share, Competitor Insights in Email Security (2026)
- Proofpoint buying Hornetsecurity in a play to expand email security scope (2025-05-15)
- Proofpoint Redefines Email and Data Security for the Agentic Workspace (2026-03-23)
- The frontier AI era demands better email security architecture (2026-06-11)