We have about 600 users and am running Exchange 5.5. Had all kinds of problems with SPAM mail going to senior executives. Have since purchased and installed XWall. It really works well and can run on the Exchange server with ver little overhead. Have had to work a bit to get everything honed down, but for $350, you can't beat it. On a daily basis, it blocks about 1100 E-Mails. Good luck!