The (IMHO) world's best spam filter, SpamAssassin, is a tricky beast to get running under Windows due to its UNIX-focused application structure. Thankfully a company has put it together with a lovely graphical interface for use on Windows and calls it No Spam Today. To make it really useful you need to add on an extra product called SpamMover which moves all messages marked as spam into a separate folder. Easy-peasy. NoSpamToday is available with a free 10-mailbox non-commercial license or you can try it for free for 30 days to see if it works for you, but unfortunately SpamMover only works with two predetermined test mailboxes before you buy it. In total you're looking at $399 for 50 mailboxes for NoSpamToday and another $100 per server for SpamMover, so $500 for 50 mailboxes, and the best thing is that there are no annual charges for either, you get free minor updates and can buy the major updates if you decide you want them, unlike most "enterprisey" software that require an annual subscription in addition to the software price. Well worth trying.