Send a templated message to one or more recipients. Provide the template once — inline in message_template with [placeholder] tokens, or as a saved template by its event key — then supply each recipient's values in template_variables.

• Set a default sender and event at the top level; override either per recipient.
• Up to 250 recipients per request.
• An unknown event, or neither event nor message_template, fails the whole request.

Response. HTTP 200 with a task_id and one entry per message. status is false when any message failed: failed entries come first and carry error (hex code) and details; accepted entries carry message_uuid (and conversation_uuid on two-way channels). Match entries by internal_id. Final delivery status is sent asynchronously to your configured webhook.

Authentication: stream token in the Authorization header (see Authentication in the API Overview).