Gateway to API Email Security
Sublime Security
This is where the legacy leaders become easier to displace, because the move from mail gateway filtering to API connected cloud protection changes both the product architecture and the buying criteria. In the gateway model, value came from sitting in front of mail flow and scanning huge volumes. In the API model, value comes from how fast a vendor can read mailbox data inside Microsoft 365 or Google Workspace, write detections, automate response, and let security teams tune rules for their own environment.
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Mimecast still has real scale, with 42,000 plus customers and 27 million end users, but its newer cloud integrated product shows the adaptation required in practice, connecting into Microsoft 365 by API and reading events from Microsoft Defender rather than only acting as the mail gateway. That is a different operating model from the one that built the company.
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Proofpoint is making the same shift. Its newer Microsoft 365 products are explicitly sold as API email security, while older deployments relied on URL rewriting and gateway inspection. That matters because API products are judged more like software inside the tenant, speed of deployment, quality of detections, and workflow automation, not just filtering volume at the perimeter.
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Sublime is built for the API era from the start. It plugs into Microsoft 365, Google Workspace, or IMAP mail servers, copies messages through journaling or Graph and Gmail APIs, converts them into structured data, and lets analysts search, write, and ship custom rules in minutes. That is the flexibility incumbents tend to lack when their products were designed first as broad security suites and channel sold gateways.
Going forward, email security keeps moving closer to the mailbox and the security analyst workflow. The winners are likely to be vendors that combine native cloud integrations, transparent detections, and automated response. That favors API native platforms, while incumbents must keep rebuilding products and go to market motions that were optimized for the gateway era.