A successful call to POST /send returns a 202 status code along with a requestId in the response body.
For send requests that have a single recipient, the requestId is assigned to the derived message as its message_id. Therefore the requestId can be supplied to the Message’s API for single recipient messages.
For send requests that have multiple recipients (accounts, audiences, lists, etc.), Courier assigns a unique id to each derived message as its message_id. Therefore the requestId cannot be supplied to the Message’s API for single-recipient messages.