Case Study - Employee #11 to 200+: Deploying Email Security to the Bank of England

The Challenge

When I joined Tessian (then called "CheckRecipient") in 2017, the company was tiny, with fewer than 15 people. They'd just raised their Series A and needed to prove they could serve enterprise clients with the most stringent security requirements imaginable.

The product was an Outlook Add-in that prevented misdirected emails (think: accidentally sending confidential client data to the wrong person). It worked great for startups, but banks don't run experimental software.

The challenge: Deploy email security infrastructure to the Bank of England and global law firms like Dentons, clients who would rather reject you than risk a breach.

What I Did

As the first client-side engineer, my job was to make Tessian's product deployable in the most locked-down environments on the planet.

On-Premise Deployments for High-Security Clients

• Built on-premise Ansible deployment scripts to install and configure Tessian in air-gapped environments
• No cloud dependencies. Everything had to run on the client's own infrastructure
• Worked directly with Bank of England IT teams to pass their security audits
• Deployed to Dentons (a global law firm) with similar requirements

Migrating the Outlook Add-in from Legacy to Modern

• The flagship product was written in VB.NET (Visual Basic .NET), a dying language
• Led the migration to modern C# to enable faster feature development
• Ensured backward compatibility with existing enterprise deployments
• Reduced bug reports by 40% post-migration

Building a High-Throughput Email Gateway

• Co-engineered a production MTA (Mail Transfer Agent) to handle millions of emails/day
• Built the full Concourse CI/CD pipeline for automated testing and deployment
• Implemented monitoring and alerting for email throughput and security events

Scaling the Engineering Team

• Helped grow the team from 15 to 200+ employees
• Interviewed and hired engineers across front-end, back-end, and infrastructure teams
• Established code review standards and engineering best practices

The Tech Stack

The Results

Tessian successfully deployed to the Bank of England, proving the product could meet the highest security standards. This unlocked enterprise deals with other tier-1 financial institutions and law firms.

The company grew from 15 to 200+ employees (backed by Sequoia, Accel, Balderton) and was later acquired by Proofpoint in a major exit.

Why It Worked

Deploying to the Bank of England isn't about clever code. It's about trust, process, and compliance.

1. On-Premise First: I built the deployment scripts with the assumption that clients wouldn't trust our cloud infrastructure. On-premise deployments gave them full control.
2. Audit-Ready from Day One: Every change was logged. Every deployment was reproducible. Every security requirement was documented.
3. Migration Without Disruption: The VB.NET → C# migration happened incrementally, with zero downtime for existing clients.
4. Concourse for Reliability: The CI/CD pipeline caught bugs before they reached production, which is non-negotiable when you're handling sensitive financial data.

Lessons Learned

Most startups think "enterprise" means "big logos and long sales cycles." That's part of it.

But the real challenge is engineering for trust. Enterprise clients don't care about your tech stack. They care about:

• Can you pass our security audit?
• Can you deploy in our on-premise environment?
• Can you guarantee uptime during business-critical hours?

At Tessian, I learned how to say "yes" to all three.

If you're building B2B software and need to prove you can serve enterprise clients, you don't need a bigger team. You need someone who's done it before.

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Want enterprise-grade infrastructure? If you need to pass a security audit, deploy on-premise, or scale your engineering team, book a call.