Challenge/Response Anti-spam Filter

I am interested in finding a company that offers Challenge/Response anti-spam filter such as www.spamarrest.com. I was going to sign up with spamarrest but discovered that they harvest your whitelist and spam your contacts. More info is at http://static.samspade.org/spamarrest.html

Does anyone know of any other challenge/response anti-spam service that does not spam your whitelist? Has anyone had an experience with using a service such as this? Pros or Cons?

Or perhaps someone knows of software I can use to setup a service such as this for myself.

Any help would be greatly appreciated.

 

 

 

 

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