Wanted: Service Looking for a free antispam solution

[DomainEmpire.com]

I'm looking for something similar to spamarrest.com but free, is there anything currently available on the market ?
I need something where all senders have to confirm themselves by clicking a link the first time in order to enter in my "white list", not for a software to install, it should be something remotely hosted exactly as spamarrest, thanks.