Catto Mail API

• Client integration guide(Markdown): authentication, idempotency, errors, pagination, validation and send jobs, webhooks, limits and quotas, polling and reconciliation.
• OpenAPI 3.1 contract(normative): every endpoint, schema and status value.

Delivery semantics: remote_accepted means the receiving mail server accepted responsibility for a message, not inbox placement; a recorded open is not a proven read; validation is not consent; an ordinary unsubscribe is not a global suppression.