Hi,
For the client's environment where i am currently implementing this, they have Zimbra as their mailing server. So it is acting as an MTA.
The mail traffic flow is that all the users send email to Zimbra (mailing server) and Zimbra subsequently after processing it forwards it to Smart Host (which is at ISP's end). So i believe Network Prevent Email has to be integrated with Zimbra for required inspection and filtering as there is no other host in between Zimbra and Smart Host.
I am planning to implement Network Prevent Email in reflecting mode.
No, they don't have Exchange Server in their environment as Zimbra is already acting as Mailing server.