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Why I'm building Posthaste

I wanted a mail client that treated my mail as mine: kept on my machine, in an open format, behind an interface I could build on — not a thin window onto someone else's server.

So Posthaste keeps a local replica of your mail and puts a documented API in front of it. The desktop app is the first client. It is not the boundary of what you can build.

It's early and the edges are sharp, but the direction is set: mail you own, shape, and program.

Best, Theo

Open source mail you can build on

Posthaste is free and open source under the MIT License. The modular backend, API, events, and MCP adapter are there to inspect, run, and build against.

Fast for daily mail. Open enough to build the workflow you want.

Shape

Views you can name and reuse.

Smart mailboxes turn a recurring boolean query into a living view, so organizing mail is something you define once and keep using.

Fast

Local state keeps the interface close.

Posthaste syncs mail metadata into SQLite, lazy-fetches bodies, and keeps previously synced mail readable offline. The interface reads from your machine instead of waiting on a round trip.

Build on

The backend is a product surface.

OpenAPI, an SSE event stream (AsyncAPI), and the MCP adapter make the local backend useful for custom clients, scripts, and trusted local agents — not just the bundled app.

Pretty, dense, and made to live in.

Compact rows, thin dividers, keyboard-first actions, and clear signal colors. A serious daily mail tool that still feels like a well-made thing — without the motion getting in your way.

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