I regularly have clients contact me in regards to controling spam and unfortunatly the answers I find never seems to get any better.
Unfortunately as business users of email with our email addresses posted on our websites it almost always seems to be a losing battle. The biggest challenge is the tighter you try to make the spam filter the more risk you run of missing an important business email that was mistakenly trapped as spam.
Barracuda Networks Releases Annual Spam Report
At Nearly 95 Percent of Email, Spam Now Rated Worst Form of Junk Advertisinghttp://www.barracudanetworks.com/ns/news_and_events/index.php?nid=232
Study: 95 percent of all e-mail sent in 2007 was spam
CNet commentary on the above press release that might be an easier read.
The following Forbes article from last year is a pretty easy read and highlights the profitability that keeps spams going. This one focuses on the specific stock market schemes that I am sure you have seen yourself in the past.
Why The SEC Can't Stop Spam