Returns the stored HTML and text bodies for a sent message. A message must have content storage enabled for this endpoint to return data. Content is available for up to 30 days after sending. A 404 indicates no content was stored for this message; a 425 indicates the content is still being stored and the request can be retried shortly; a 410 indicates the content was stored but has since expired. ID of the message whose stored content to fetch, from the send response's `id` field (`em_`-prefixed). The HTML body of the message, if it was stored. The plain-text body of the message, if it was stored.