SMS; TxtStream; When replying to an SMS generated from a Document, the Email referred to "Unknown" rather than the Account Id and Name

When sending a TxtStream SMS via a Document (e.g., an Account SMS Advice), if the Client replied to the SMS, the Email referred to "Unknown" rather than the Account Id and Name, e.g.:

This is in response to a message sent to Unknown (+6421XXXXXXX) on 11 Aug 12:47