Basically a setup like this should work well:
combined with these additional postfix settings:
You can use this setup e.g. as dedicated spam server in front of your normal mailservers by forwarding the emails after the scanning process with postfix transports.
Another way is to use ispconfig 3
as it sets up the virtual server setup automatically during installation and offers a easy to use interface and the ability to maintain different antispam scorings and per domain or even per email address, you will just have to add razor, pyzor, dcc and the blacklists as described in the howtos above.