SolMail: Micropayments for cold emails on Solana

Use the oldest release v1 apk to try out Solmail on Solana Seeker phone

An AI-powered email client built on Zero that sends micropayments upfront and refunds them if replies aren't meaningful, ensuring you pay solely for successful conversations.

Table of Contents

• Hackathon Submission (EasyA Consensus)
• About SolMail
  • Problem
  • Solution
  • Key Features
• Getting Started
• Project Structure
• Technology Stack
• Contributing
• License
• Links
• Support

Hackathon Submission (EasyA Consensus)

• Short summary (<150 chars): Gives Seeker users paid email on Solana, releasing escrowed SOL only when the recipient sends a real reply judged by an on-device agent.
• Download APK: solmail-consensus-v1.apk
• Canva slides: view
• Demo video (with audio): YouTube
• Screenshots: see section below
• Technical description (SDKs + sponsor tech): see section below
• Blockchain interaction explanation: see section below

Screenshots

Drawer	Compose panel	Inbox list

Wallet connect	Wallet approval	Wallet transaction

Technical description

SolMail is a native Android Solana dApp built on three pieces of sponsor tech: an Anchor escrow program, Mobile Wallet Adapter with Seed Vault on Seeker, and solana-agent-kit for autonomous refunds. Backend on Cloudflare Workers, on top of the open-source Zero email framework.

Anchor program. create_escrow, release, withhold over a PDA seeded on ["escrow", msg_id], so every email thread has a deterministic escrow. The mobile client builds raw TransactionInstructions using 8-byte Anchor discriminators baked into compose.tsx, so the APK ships without the Anchor TypeScript SDK on-device.

MWA and Seed Vault. Every send and claim is signed through @solana-mobile/mobile-wallet-adapter-protocol-web3js. transact() in mwa.ts routes to Seed Vault on Seeker, where the key never leaves the secure element and the user gets a native biometric prompt, and falls back to Phantom or Solflare on any other Android device. One code path, two custody models, zero Seeker-specific code.

Autonomous refund agent. SolanaAgentKit boots with a KeypairWallet from env. processEmailReply runs score, decide, then releaseEscrowAction or withholdEscrowAction, which call program.methods.release() and withhold() via Anchor server-side. Required because no human would sign their own refund.

Reply scoring. LangChain ChatOpenAI returns 80 for genuine replies, 5 for empty, gibberish, spam, or prompt injection. decide() enforces score >= 15, server-side. Client sees only a boolean.

Cloudflare stack. tRPC on Workers with six Durable Objects, Hyperdrive Postgres, Workers AI, Vectorize, R2, Queues, KV. Escrow settlement runs in ctx.executionCtx.waitUntil(), never blocking send. Escrow metadata rides in X-Solmail-Sender-Pubkey and X-Solmail-Thread-Id headers, so messages stay valid SMTP.

Uniquely enabled. Seed Vault gives hardware key custody with no custom code. MWA ships one APK to Seeker and every other Android wallet. solana-agent-kit lets the server hold its own Solana wallet without ever touching user keys.

Blockchain interaction

1. Send. The mobile app signs an init_escrow transaction via Mobile Wallet Adapter. On Seeker this routes to Seed Vault for biometric approval. The instruction creates an escrow PDA seeded by ["escrow", random_32_byte_thread_id] and locks the sender's SOL on devnet.
2. Carry. The 32-byte thread ID and the sender pubkey are embedded in the outgoing email as X-Solmail-Thread-Id and X-Solmail-Sender-Pubkey headers, keeping the message valid SMTP so it reaches plain Gmail too.
3. Reply. The recipient opens the thread in the SolMail app. Hitting Send triggers a server-side reply-quality check. If the check passes, the app signs a register_and_claim instruction that closes the escrow PDA and transfers the lamports to the recipient's wallet.
4. Refund. After every reply, a Cloudflare Worker async job (processEmailReply in escrow-agent.ts) re-runs scoring authoritatively. On a fail verdict, the server's solana-agent-kit keypair signs withhold to refund the sender. No user signature required.
5. Settlement. All four transactions (init_escrow, register_and_claim, withhold, plus account creation) are visible on the Solana devnet explorer and signed by hardware-backed keys on Seeker, or by Phantom/Solflare on other Android devices.

About SolMail

SolMail is an innovative email client that revolutionizes cold emailing by implementing a pay-for-success model. Built on the Zero email framework, SolMail ensures you only pay for meaningful email conversations.

Problem

Cold emailing is inefficient and costly. You pay for emails that don't get meaningful replies, wasting resources on unsuccessful outreach campaigns.

Solution

SolMail sends micropayments upfront and refunds them if replies aren't meaningful, ensuring you pay solely for successful conversations that drive real business value.

Key Features

• Pay for Success - Send micropayments upfront, get refunds if replies aren't meaningful
• AI-Powered - Leverage AI to enhance your email experience
• Built on Zero - Powered by the open-source Zero email framework
• Privacy First - Your emails, your data
• Fast & Reliable - Lightning-fast interface and reliable delivery

Getting Started

SolMail has two parts that run side by side: the Zero backend (Cloudflare Workers + Postgres) and the Android app (Expo / React Native). You need both running for the mobile app to work end-to-end.

Prerequisites

• Node.js + pnpm (v10+)
• Docker (for the local Postgres + Redis containers)
• Android Studio + Android SDK (only needed for the first build)
• A physical Android phone with USB debugging enabled, plugged in via USB

1. Backend — Zero

In one terminal:

cd Zero
pnpm install
cp .env.example .env       # then fill in real values
pnpm docker:db:up          # starts Postgres on :5432, Redis on :6379, Upstash proxy on :8079
pnpm dev                   # backend on http://localhost:8787, web app on :3000

Leave this terminal running. The backend hot-reloads on file changes.

2. Android app — solmail-android

In a second terminal:

cd solmail-android
pnpm install
cp .env.example .env       # uses EXPO_PUBLIC_BACKEND_URL=http://localhost:8787 (works via adb reverse)
pnpm android --device      # first time: builds APK, installs, launches; ~5–10 min

pnpm android builds the native APK and exits — Metro bundler runs separately. After the first build, day-to-day development looks like:

cd solmail-android
pnpm start                 # starts Metro + auto-runs adb reverse for ports 3000/8081/8787

Then save any .tsx file → Fast Refresh updates the app on your phone instantly. You only need to re-run pnpm android when you add a native dependency or change solmail-android/app.json.

The start and android scripts automatically run adb reverse tcp:3000, tcp:8787, and tcp:8081, so the phone reaches the Mac via USB tunnel — no Wi-Fi config needed. The phone must be plugged in.

Project Structure

solmail-android/
├── Zero/                  # Zero email backend + web app (Cloudflare Workers, Postgres)
│   └── apps/
│       ├── mail/          # React web client (port 3000)
│       └── server/        # tRPC + Better Auth backend (port 8787)
├── solmail-android/       # Expo / React Native mobile app
│   ├── app/               # Expo Router screens (.tsx)
│   ├── src/               # Wallet, auth, tRPC client
│   └── android/           # Native Android project (generated by `expo prebuild`)
├── escrow/                # Solana escrow program (Anchor)
└── solidity-escrow/       # EVM-side escrow contracts

Technology Stack

SolMail inherits the robust technology stack from Zero:

• Frontend: React, TypeScript, TailwindCSS, React Router
• Backend: Node.js, Drizzle ORM
• Database: PostgreSQL
• Authentication: Better Auth, Google OAuth
• Deployment: Cloudflare Workers

License

This project is licensed under the same license as the Zero framework. See the LICENSE file for details.

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Built with Zero