So how do spammers get your email address?
I’ve already mentioned email harvesting. Where a spammer uses a computer program to spider (surf) the web and collect any valid email addresses they can find. They also do this on usenet as well.
The Whois database is another way for people to grab email addresses. They look at who owns the domain name and collect the email addresses that way.
Some companies will sell your email address after you have bought items from them. This is more common then people want to know about.
I have even seen evidence of major online email services selling the email addresses on the non-members who send email through their system. Meaning that if you send email to a member from outside the system they can collect your email address and sell it to interested people. Remember, they have privacy agreement with this member, not with the people who email their members.
I do believe that many spammers use viruses to do this dirty work for them. They will setup a honeypot computer and and few email accounts and let the computer get infected with an email worm, or type of virus, then collect the data about email addresses that the worm collects as it tries to send itself across the web.
Those are just a few off the top of my head. If you think of anymore please email me or post them on the forums.