If you’re evaluating Microsoft Defender for Office 365 for email security software, the three strongest independent alternatives in our editorial ranking are Abnormal Security, Proofpoint, Mimecast. Each has a different best-fit buyer — the right choice depends on team size and workflow, not on which has the loudest review-site presence.
Why Microsoft Defender for Office 365 sometimes isn’t the right pick: Google Workspace organizations (Defender does not protect Google Workspace), buyers prioritizing best-in-class behavioral AI for BEC/ATO (Abnormal wins as overlay), or non-Microsoft enterprises generally. See full “worst for” verdict →
9 Microsoft Defender for Office 365 alternatives
| Rank | Product | Best for | Target size | Pricing |
|---|---|---|---|---|
| #1 | Abnormal Security | Enterprises (1,000-100,000+ employees) on Microsoft 365 or Google Workspace adding an AI-behavioral overlay on top of Defender for O365 or a legacy SEG to catch BEC, ATO, and vendor email compromise. | 500–100,000+ | ○ Quote-only |
| #2 | Proofpoint | Large regulated enterprises (financial services, healthcare, federal) wanting a mature SEG with the broadest archive / DLP / compliance bundle and willing to pay premium pricing under PE ownership. | 1,000–500,000+ | ○ Quote-only |
| #3 | Mimecast | Mid-market and enterprise buyers (500-50,000 employees) wanting an integrated bundle of email security + continuity + archive + awareness training under a single vendor, particularly in the UK and EU. | 500–50,000+ | ○ Quote-only |
| #5 | Cisco Secure Email | Enterprises already committed to Cisco network security stacks (Firepower, Umbrella, SecureX, Duo) wanting unified security with Talos threat intelligence and the option of on-prem appliance deployment. | 1,000–500,000+ | ○ Quote-only |
| #6 | Barracuda Email Protection | Mid-market organizations (200-5,000 employees) wanting integrated email security + backup + archive + awareness training under a single mid-market-priced vendor, often through MSPs. | 200–10,000 | ◐ Partial |
| #7 | Avanan | Organizations on Check Point Infinity / Harmony stacks (firewall, endpoint, SASE) wanting unified email security as part of a single Check Point platform. | 500–50,000+ | ○ Quote-only |
| #8 | Sublime Security | Detection-engineering security teams (200-20,000 employees) that already write custom Sigma, YARA, or Snort rules and want the same level of control and transparency over email detection logic. | 50–20,000 | ◐ Partial |
| #9 | Material Security | Security-mature organizations (1,000-50,000 employees) that have accepted that mailboxes will eventually be compromised and want to limit the blast radius, particularly financial services, legal, and high-IP-value verticals. | 500–50,000 | ◐ Partial |
| #10 | Vade | European mid-market organizations (200-10,000 employees) prioritizing French / European data residency, and ISPs / telcos protecting consumer and SMB mailboxes at scale via OEM. | 200–10,000 | ◐ Partial |
Which alternative for which buyer
Abnormal Security
AI-driven behavioral anomaly detection, modern category leader for BEC, ATO, and vendor email compromise.
Enterprises (1,000-100,000+ employees) on Microsoft 365 or Google Workspace adding an AI-behavioral overlay on top of Defender for O365 or a legacy SEG to catch BEC, ATO, and vendor email compromise.
Cost-sensitive SMBs (Defender for Business or Microsoft 365 Business Premium cheaper), on-prem Exchange shops (unsupported), or buyers wanting a single replacement gateway rather than an additive overlay.
Proofpoint
Largest legacy enterprise SEG installed base; Tessian acquisition added behavioral AI.
Large regulated enterprises (financial services, healthcare, federal) wanting a mature SEG with the broadest archive / DLP / compliance bundle and willing to pay premium pricing under PE ownership.
Microsoft 365 E5 shops looking to consolidate (Defender for O365 wins), cost-sensitive mid-market (Barracuda or Defender cheaper), or modern AI-behavioral leadership buyers (Abnormal wins on detection).
Mimecast
Legacy SEG with mature archiving and awareness training bundle; PE-owned post-2022.
Mid-market and enterprise buyers (500-50,000 employees) wanting an integrated bundle of email security + continuity + archive + awareness training under a single vendor, particularly in the UK and EU.
Microsoft 365 E5 shops consolidating (Defender for O365 wins), modern AI-behavioral leadership buyers (Abnormal wins), or buyers wanting transparent pricing.
Cisco Secure Email
IronPort heritage; tight Cisco SecureX and Talos threat intelligence integration.
Enterprises already committed to Cisco network security stacks (Firepower, Umbrella, SecureX, Duo) wanting unified security with Talos threat intelligence and the option of on-prem appliance deployment.
Non-Cisco shops (Defender for O365 or Abnormal better), modern cloud-native organizations (any of the modern AI-behavioral leaders win), or buyers wanting transparent pricing.
Barracuda Email Protection
Mid-market email + backup + archive bundle; KKR-owned post-2022.
Mid-market organizations (200-5,000 employees) wanting integrated email security + backup + archive + awareness training under a single mid-market-priced vendor, often through MSPs.
Large enterprises (10,000+ employees) where Proofpoint, Mimecast, or Defender win, modern AI-behavioral leadership buyers (Abnormal wins), or buyers wanting transparent pricing.
Avanan
Cloud-anchored API-integrated email security; tight Check Point Infinity integration.
Organizations on Check Point Infinity / Harmony stacks (firewall, endpoint, SASE) wanting unified email security as part of a single Check Point platform.
Non-Check Point shops (Defender for O365 or Abnormal better), modern AI-behavioral leadership buyers (Abnormal wins), or buyers wanting transparent pricing.
Related editorial
- Full Top 10 Email Security Software for 2026 ranking with comparison table and decision matrix →
- Who shouldn’t buy Microsoft Defender for Office 365? Editorial “worst for” verdict →
- Microsoft Defender for Office 365 vendor trust score (6 dimensions, dated) →
- Microsoft Defender for Office 365 full intelligence profile →
Last updated 2026-05-09. Rankings reflect editorial judgment based on the published Top 10 Email Security Software for 2026. We accept no vendor payments. Found something inaccurate? Tell us.