Exchange Compatible Spam Filters?

DamienMcKenna asks: "At work our license for Symantec Brightmail is coming up for renewal and I'm looking for alternatives that will cooperate with Microsoft Exchange 2003. Brightmail hasn't worked consistently since we installed it last year, has a low success rate, the client plugin has been very unstable, and it takes up far too much server resources for what it does. Given that many of the appropriate software is not available for trial (you have to base decisions off their marketing materials), does anyone have recommendations on what to use instead? It must be Windows-based (UNIX/Linux/BSD is out of the question right now), and should have an easy to use administrative interface since not all of the IT staff are very technically minded. A working plugin for Outlook for client-level configuration would also be appreciated."