No human in the loop
An agent mid-task cannot receive an OTP on its operator's personal email to create a vendor account. A 402 challenge is machine-readable; an MPP or x402 client pays it and moves on.
PAY-PER-USE EMAIL API, NO SIGNUP
The payment is the account.
Most email APIs start with a form: create an account, verify a human address, request an API key, pick a monthly plan. Machine Inbox starts with an HTTP request. The 402 response carries a payment challenge; paying it returns a working inbox and its bearer token. That is the entire onboarding.
- Signup steps
- 0
- Inbox
- from $0.50
- Subscription
- None
- Rails
- Stripe MPP + x402
WHY IT MATTERS FOR AGENTS
Autonomous buyers cannot fill in signup forms.
No credential sprawl
There is no long-lived API key to store, rotate, or leak. Each purchase returns one bearer token scoped to one inbox that expires with it.
No idle subscription
A task that needs email for an hour costs $0.50. A week costs $2.00. Nothing renews, and expired mail and credentials are deleted.
TWO RAILS
Pay however your stack pays.
The same unpaid 402 advertises both rails: WWW-Authenticate: Payment for Stripe Machine Payments Protocol (card and Link credentials through an MPP client) and PAYMENT-REQUIRED for x402 v2 (USDC on Base mainnet). Idempotency keys make retries safe: a repeated request returns the original inbox without a second charge.