One other way that your emails get added to spam lists is from actual people browsing websites looking for them. I know this happens as well. There's so many different ways that spammers can get a hold of your email address. Another one is companies selling them. And yet another is people going through whois information and gathering them.
If you just don't give your email out to anyone, except for those you want to have it, don't subscribe to anything, and make sure you read all the terms of service and privacy policies for all the things that you do sign up with that require your email and make sure they don't do any of the above then you should be safe for the most part. But that's not saying that it will protect you 100%.
Oh yea, just thought of another way that spammers may get your email address. You know when your friends have you in their address book and they get some email that they think everyone in their address book should have? They send it to everyone in their address book, and everyone that got it, sends it to everyone in their address book, and it just continues on and on. So it's very possible that one of the people that get the email is a spammer themselves or gathers the email addresses for the soul purpouse of selling them to spammers.
