Hi,
I understand that you not need Antispam & Firewall, but if you want to use outlook in configuration with your Exchange server I presume you must buy the BPP for all the clients you need.
I think you can’t buy a license of BPP only for the Exchange server, you must buy a number of license equal to number of Pc connected to Exchange server…
I think your local reseller can explain better.
http://www.avast.com/locate-dealer