I have two filters, both from clearswift. My first filter is on a stand alone SMTP Gateway server It is specific filter for SPAM Prevention which also scans for viruses using a Sophos definition package. The other is "Mailsweeper for Exchange 2000", installed locally on my exchange server, it's mostly for keyword filters. I use it to strip all executable attachments, (zip,pif,bat,exe,....etc) . and to prevent corporate espionage... my boss uses it to spy on people...

but yeah, if you're the admin, there's no perfect filter that will guaruntee that it will let all real mail come through and destroy all SPAM and virus mail... This forces the admin to dig through everything regardless of what's caught, to make sure that no real e-mails are lost by digging through every single e-mail..... which in a company of 100 people... 10 of which it seems love to toss their business e-mail address across the internet and put themselves on every SPAM lists immaginable adds up to thousands...

If you are your own admin, you might as well not even have a filter and just use the block feature in your e-mail client and add all the common Spam sources to it. You're gonna have to go through it all either way, might as well save your money.