"The critical factor for me is the absence of
false positives. Thunderbird spam-filtering, Spamato, Vipul etc etc all
catch an occasional genuine email -- which make them effectively
unusable for me. Cloudmark for Outlook, powered by Vipul's Razor, was
pretty good, but still let through loads of spam. You, by contrast, seem
to have processes which effectively turn off the spam faucet."

Thomas Nichols, nexus10.com