Start sending now. The campaign must be draft or error; it moves to running, started_at is stamped, and the contact list is fixed as of that instant — contacts added to the list afterwards are not messaged by this run.
Sending then proceeds in the background, one message per contact in that snapshot. It reaches completed once every message it created has settled, or error if the run stops first. Launching an error campaign again retries the run without messaging a recipient it already messaged.
Returns 409 conflict with error.code = "invalid_state" when the campaign is not in a launchable state, or error.code = "import_in_progress" while a bulk contact load is still running on the target list.
Launch accepted: the campaign is now running and sending proceeds in the background; no body is returned. Accepted means started, not sent.