Mail merge inside Outlook is stuck in the past. The built-in feature can’t attach a different file to each recipient, and the third-party tools that can will route your contacts and message content through their own servers to do it. On top of that, “new Outlook” — Microsoft’s rebuilt Windows, Mac, and web client — broke most of the legacy COM add-ins people relied on.

So anyone who needed to send two hundred personalized invoices, each with its own PDF, straight from their own mailbox was left without a good option that also respected their data. That gap was the brief: a genuinely cross-platform mail-merge tool where privacy is the architecture, not a marketing line.

We designed and built MergeBird end-to-end as a single cross-platform Office.js add-in. The task pane is a React + TypeScript + Fluent UI wizard — write the template, import recipients from CSV or Excel, attach per-recipient files, preview every merge, then send. Because Office.js itself can’t send a batch of fresh emails, delivery runs through Microsoft Graph’s /me/sendMail, authenticated with Nested App Authentication so the user signs in once inside Outlook with no redirect dance.

The hero feature — a different attachment for every recipient — matches uploaded files to a spreadsheet column by file name, entirely in the browser. That’s also what makes the privacy promise real: recipients, message bodies, and attachments never touch our infrastructure. A deliberately small Hono backend handles only what it must — anonymous license/quota counts and Stripe billing — and is the only server in the picture.

Around the core we built the rest of a real product: a Trust Cobalt brand and logo, a marketing site, honest opt-in analytics (aggregate-only open and click tracking, no per-recipient data), live Stripe subscriptions and credit packs, and a one-command deploy to DigitalOcean behind HTTPS with auto-renewing certificates — plus a validated AppSource submission package.

MergeBird shipped as a complete, monetized product. One codebase runs across new Outlook on Windows, Mac, and the web; real campaigns send from the user’s own Microsoft 365 mailbox; and Stripe billing is live in production. Because the merge happens client-side, the backend only ever sees an opaque key and a send count — never a recipient, a subject line, or an attachment.

The result is a small, fast, fixed-scope SaaS that demonstrates the full path Black Hart Consulting can take a product down: discovery, design and branding, full-stack engineering against a hard third-party platform, billing, infrastructure, and marketplace submission — all the way to a live, paid product.