InternetWeek:

"Cloudmark, the developer of SpamNet, a community approach to sniffing out junk e-mail, has released an enterprise spam killer engine that identifies unwanted messages by their structure -- or spam DNA -- rather than by their domains or keywords. Authority, as the engine's dubbed, blocks spam at the gateway, preventing junk mail from entering a company's network and eating up precious employee productivity.

[...] Cloudmark relied on its experience with SpamNet, a peer-to-peer network of more than 200,000 users who report spam to a central database, to identify probable spam structures."

It makes sense for them to provide free copies to individuals (I've been running Cloudmark since about when it's been made available to the public). In exchange I do some manual work to tag unflagged spam that got through their filter.