As Keith pointed out SMTP Prevent is not an MTA, think of it as an MTA proxy.
The process is the following:
Sending MTA connects to SMTP Prevent (Prevent)
SMTP Prevent connects to forwarding MTA
Prevent receives the email from the sending MTA and immediately sends it to the forwarding MTA
When the sending MTA signals the email is complete, Prevent processes the email.
If the email is not blocked, Prevent completes the process with the forwarding MTA.
If the email is blocked, Prevent aborts the email with the forwarding MTA and fails the email back to the sending MTA.
Based on the failure code, the sending MTA does predefined actions on the email, including sending the email back to the sender.
JGT