The main use case I kept running into while building: automated testing of sign-up/verification flows. You need a real inbox, a real OTP, and you need it fast. Existing options were either not API-first, had terrible docs, or charged $50/mo before you could even try them.

What it does: - POST /v1/inboxes to register an inbox - GET /v1/inboxes/:inbox/otp to pull the latest OTP (parsed automatically) - WebSocket connection for real-time push instead of polling - Attachments, custom domains on higher tiers

Auth is SHA-256 hashed API keys, usage tracked per-user (not per-key, so rotating keys doesn't reset your quota). Rate limit headers on every response.

Free tier: 5k req/mo, no credit card.

Interactive playground: https://www.freecustom.email/api/playground Docs: https://freecustom.email/api/docs

Happy to answer questions about the architecture — running on Node/Express, Redis for rate limiting and pub/sub, MongoDB for storage.