In my experience spamers use mostly email addreses publicly exposed (web sites, usenet, forums) and stolen address books (viruses, malware) - you can't do much about the second if that happens to your recipients.
Hi imhoguy, spammers also happen to buy leaked data sometimes to better target people. Anyway, spam is one thing we address, but that's not the main point. It's more about control of your privacy.