I don't know quite how high a performance you need. If it's just email
spam/viruses you are concerned with, you can run MailScanner for free,
see http://www.mailscanner.info. It's been around for 10 years now and
used by a lot of big organisations, many of which are listed on the
web site. Written by a colleague here at University of Southampton,
hence the plug. If you install and run it yourself, there's a good
community mail list for support and tips.