post https://sandbox.dmhisp.com/SecureMessagingApi/Message
This method sends a message to a given recipient(s) from the user associated with the SessionKey.
The maximum supported JSON payload for this method is 15MB. This size limit includes your message and Base64-encoded attachments, so we recommend keeping your total attachment size around 10MB or less to account for the ~30% increase in payload size from the Base64 attachment encoding.