There are a number of ways spammers harvest your email address. Here are some of the more common ways:
Message boards
This has got to be the number 1 favorite! Spammers have automated software that follows each and every link on a message board looking for email addresses. When you sign up at a Message Board, make sure you set your email address to Private! (It differs on various Message Boards but most have this option. If the Board software does not allow this, don't join it!)
Also, check the board's privacy policy. If they don't have one, write to the owners and ask. They must give you assurances that they do not sell email addresses. Believe it or not, there is money to be made in selling "targeted" lists of email addresses. Some message board owners make bug bucks selling your email address to the spammers! For example, some companies pay up to $0.50 per email address. Take a message Board with 3,000 members that's $1,500 each time he sells the list (Oh yes, some sell the lists over and over so you land up on not one but hundreds of SPAM lists!!).
Guest Books
The same rules apply to guest books as to Message Boards. Never put your email address in a guestbook unless the email address is either "munged" (obfuscated) or not made visible to the general public. Again, check with the owner of the guest book first before you reveal your email address to the world.
Forwarding email
Do you regularly get jokes and interesting articles sent to you by friends? Do you forward these to all your other friends? What do you think your friends do? Yeah, right, they forward them too, and so do their friends, and so do their friends, and so do ... get the picture? Now, here's the $64,000 question. Do you clean up the email (remove the email addresses of the people before you) before you send it? Do the friends you send it to remove your email address? Yeah, right that's what I thought. ;)
What this means is that within minutes, your email address could be in the inbox of thousands of other people you don't know from a bar of soap. If only one of them is a spammer your email address is toast!
"But my friends are not spammers" you might say. Man! Have I got news for you! First, no spammer goes around with a banner on his back saying "Kick me, I'm a spammer". Spammers are your colleagues, secretaries, friends, teachers and even family! They come from every walk of life. They don't think (some don't even know) what they are doing is illegal (Oh yes, SPAM is illegal by the way. People can loose their jobs and even serve time for for it). Most believe it's OK because they are running a little business on the side making a few bucks every month.
An interesting story. I recently set up a sting on a "friend" I suspected was a spammer. I set up an email address at hotmail and then forwarded him some "cool jokes" to him from there. Within 24 hours the hotmail email address was being spammed by tons of junk mail. I later discovered, the hotmail address had been sold (along with hundreds of others) to over 400 SPAM mail companies! Some friend!