"Block SMTP Email" sending bounce message downstream

DLP 11.6.3 on RHEL

Network Prevent (Email) in Forwarding Mode

Exchange MTA --> DLP Prevent Server --> MessageLabs

I am trying to test blocking of SMTP emails and I have added a "Bounce Message to Sender".  But that message is being sent to my downstream MTA instead of the sender.  According to the Prevent_MTA_Integration Guide:

"This rule blocks the message by returning a 550SMTPmessage to the Prevent-integrated MTA or to the upstream MTA."

How do I get this to be returned to the upstream MTA instead of the downstream MTA (MessageLabs)?