From Blue Squirrel:
Spam Sleuth Enterprise eliminates spam for your entire organization. The perfect anti-spam solution for ISPs, businesses, schools, governments and other organizations. Works with any e-mail server.
Spam Sleuth Enterprise resides on your e-mail server, or on a computer next to your e-mail server and screens out spam, viruses, Phishing scams, and Web Bugs before they reach your e-mail server. Includes a PC/Web client for users to configure spam settings and view spam messages. Users will receive e-mail the same way they always have they will just notice a lack of spam, and increase in productivity.
e-mail stamps: Charge senders a fee to send e-mail. When Spam Sleuth deems an e-mail message as spam an email sent back to the sender requiring them to pay a fee before their message will be sent through to an inbox.
Turing test: Similar to Email stamps, except that you do nott collect money. The turning test sends an email back to the sender and requires them to enter a validation code in order for Spam Sleuth to allow email messages through to your inbox.