I'll have to look more closely at this one because it might be HPP dependent. Basically, in order to prevent unauthorized communications, some services (such as paypal's chained payment within a HPP) takes the response header from the HPP and immediately sends it back to Paypal to verify that it did in fact come from paypal. Looking at authorize.net's SIM, this validation may not always be the case. Therefore, I will modify this to specify that the HPP sends a response that contains all the relevant confirmation information, and that may be sent back to the HPP to confirm the transaction came from the intended gateway.