Best Email Security Software in 2026: API and Gateway Tools Compared

Best for: Microsoft 365 customers who want stronger phishing and attachment protection without adding a second vendor or console.

• Lowest published entry price on this page at $2.00 per user per month for Plan 1, billed yearly.
• Plan 2 bundles attack simulation training and automated investigation, which can remove the need for a separate phishing simulation tool.
• It only protects Microsoft 365, and its policy model is complex enough that many smaller tenants run it with default settings that leave gaps.

Our take: Defender for Office 365 is the default starting point for any company already on Microsoft 365, because it plugs into the same admin center, the same identities and the same alerting pipeline you already use. Plan 1 covers Safe Links, Safe Attachments and protection for internal email and Teams, while Plan 2 adds the investigation and simulation tools that a security team needs. Many companies already own it through Microsoft 365 E5 or Business Premium bundles, so check your licenses before buying anything. In a trial, review the preset security policies, test how quarantine notifications reach users, and measure how many BEC-style messages with no link or attachment still reach inboxes. If gaps remain, an API-based layer such as Sublime, Ironscales or Abnormal can sit on top of Defender rather than replace it, which is a common pattern for mid-market and enterprise buyers on this page.

Best for: Security teams that want transparent, editable detection logic and a free starting tier for smaller mailbox counts.

• Official documentation states the managed option covers the first 100 mailboxes free, which lets small teams run it in production at no cost.
• Detection logic is visible and editable in MQL, so analysts can see exactly why a message was flagged and write their own rules.
• Pricing above the free allowance is not published, and getting full value requires someone who will read, tune and write detection rules.

Our take: Sublime takes a different approach from most email security vendors: instead of a black-box model, detections are rules written in its Message Query Language and shared through community feeds, and you can inspect or modify every one of them. That suits security teams that already run detection engineering for endpoints or cloud logs and want the same discipline for email. Deployment is flexible, from the managed cloud service to self-hosting on Docker, AWS or Azure, which matters for organizations with data residency rules. The free allowance for the first 100 mailboxes on the managed option makes it easy to test against real mail flow. In a pilot, compare its catches with what Defender or Google already blocks, check how much rule tuning your team needs to do each week, and ask for written pricing for your full mailbox count before you commit.

Best for: Mid-market IT teams and MSPs who want API-based phishing protection plus optional awareness training in one product.

• Official pricing page lists separate business and MSP plan lines, including Complete Protect with integrated security awareness training and unlimited phishing simulation.
• Deploys via API with no MX record changes and can coexist with a secure email gateway.
• Prices are not published, so you need a quote to compare it with the per-user list prices from Microsoft.

Our take: Ironscales combines automated email threat detection with a strong user reporting loop: employees flag suspicious messages, the platform clusters similar emails across mailboxes and can remove them automatically. That design helps lean IT teams that do not have a dedicated SOC analyst reviewing every report. The plan ladder runs from Email Essentials with phishing and BEC detection up to Complete Protect, which adds account takeover protection and security awareness training with unlimited phishing simulation, so you can consolidate two tools into one contract. MSPs get their own plan line. During the 14-day trial, connect it in read-only or detect mode first, compare its catches against your current filter, and test how quickly reported phishing is pulled from other inboxes. Compared with Abnormal, it is more accessible to mid-market buyers; compared with Sublime, it requires less rule work.

Best for: Companies that need one API-based layer protecting email and collaboration apps such as Teams, Slack, SharePoint and Google Drive.

• Covers Exchange Online, Gmail, OneDrive, SharePoint, Google Drive, Box, Dropbox, Teams and Slack according to Check Point admin documentation.
• Inline API mode inspects messages before they reach the inbox rather than only removing them afterward.
• Minimum seat licenses apply and the package mix of bundles and add-ons makes quote comparison more work.

Our take: Harmony Email & Collaboration began as Avanan, which Check Point acquired in 2021, and it remains an API-first product that connects to Microsoft 365 and Google Workspace in minutes. Its main difference from the other API tools on this page is breadth: the same policies and console extend to file sharing and messaging apps, so a malicious link dropped into Slack or a poisoned file in SharePoint gets the same treatment as an email. The inline mode can hold messages for inspection before delivery, which some buyers prefer to post-delivery clawback. In the 14-day trial, check which add-ons you actually need, because DLP, encryption and sandboxing are offered a la carte, and confirm the minimum seat count for your size. Companies already using Check Point firewalls or Harmony endpoint products gain the most from shared management and threat intelligence.

Best for: Large organizations where vendor fraud, executive impersonation and account takeover are the main email risks.

• Reads identity, behavioral and content signals, including authentication events and internal threads that a gateway cannot see.
• Product line extends to account takeover protection, AI phishing coaching and misdirected email prevention.
• There is no published price or self-serve trial, so evaluation runs through a sales-led proof of value.

Our take: Abnormal AI, which rebranded from Abnormal Security, is built around the idea that the hardest email attacks look normal: no malware, no bad link, just a convincing request from a real or spoofed supplier. Its models learn how each employee and vendor normally communicates, then flag invoices, bank detail changes and urgent requests that do not fit that pattern. Because it connects via API, it can also see sign-in events and internal mail that a gateway in front of the mailbox never sees. The proof of value process runs it alongside your existing tools and reports the attacks your current setup missed, which gives you hard numbers for a business case. Ask for per-mailbox pricing at your size, check how verdicts are explained to analysts, and confirm how vendor email compromise cases are handled. It sits at the enterprise end of this list together with Proofpoint.

Best for: Enterprises that want gateway and API protection from one vendor, with optional DLP, fraud defense and awareness training.

• Supports inline gateway, API-based and combined deployment, covering pre-delivery, in-mailbox, post-delivery and click-time protection.
• Adjacent modules include Email Fraud Defense for lookalike domains and DMARC, Adaptive Email DLP and ZenGuide awareness training.
• No published pricing or trial, and the breadth of modules means scoping takes time.

Our take: Proofpoint is a long-standing choice for large organizations that route mail through a secure email gateway and want the option to add API-based detection for Microsoft 365 on top. Core Email Protection can run in either mode or both, which suits companies with hybrid or on-premises mail, complex routing rules, or regulatory requirements for inline filtering. Its ecosystem is the broadest on this page: DMARC and lookalike domain monitoring, outbound DLP, account takeover detection and the ZenGuide training platform all sit in the same family. The tradeoff is cost and complexity, so scope carefully. In a demo, ask which modules are needed for your risk profile, how the API and gateway layers share verdicts, and what professional services are included. Smaller companies usually get better value from Defender, Ironscales or Harmony; Proofpoint fits when scale and routing complexity are high.