Tim Chown (tjc) writes:
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.
... or just run amavisd. MailScanner used to do Bad Things with the Postfix queue, but since then I think they have fixed that, but I will admit to not having any experience with it. As to amavisd: http://www.ijs.si/software/amavisd/ Have been using it on 1 million mails / day with satisfaction