What is public
Authenticate with Authorization: Bearer fm_live_…. The contract is OpenAPI 3.1 at /openapi.json on the API host.
GETandPOST /api/v1/inboxesGETandPATCH /api/v1/messagesandGET /api/v1/messages/:idGET /api/v1/searchPOST /api/v1/emailsfor transactional send
The same scopes and mailbox limits as Agent Keys apply. A send that needs approval returns 202 with status: approval_required and does not send.