I have 5 Linux Plesk 12 servers, and I use Spamassassin, usually at a sensitivity of 2 or 3, I also use the following DNS Blackhole lists:
zen.spamhaus.org;b.barracudacentral.org;abuse.rfc-ignorant.org;cbl.abuseat.org;bl.spamcop.net;nomail.rhsbl.sorbs.net
But all of this seems to have minimal...