Other methods are more malicious and involve virus-like or worm software being installed on your computer and feeding the names in the address book back to a spam server that collects them.

Spammers trade email addresses for money. This is why the system is out of hand - it is an income producing activity. Every email address has a value to it and no matter how little the value, putting together a list of 100,000 can provide a neat income for a spammer. Most spammers have spam lists many times larger than this.

Spammers also include the unsubscribe link at the bottom of spam emails. These links when clicked and actioned, tells the spammers that this email address actually got through to a live person and that email address is now moved onto a much higher value list and is traded for more money. Those of us who have actioned these links have found the flood of spam coming in to vastly increase within only days.

So now to the question of how to use this knowledge to reduce the spam you get?

First, understand that everytime you give out your email address to a website, that it can potentally be sold and traded. Therefore, if you are unsure about it, use an email address that is not your primary one. Webmail services like Hotmail and Yahoo Mail are perfect for this. You only get the messages when you want them and log into those services. I've used a Yahoo Mail address for this for years now. Everytime I log in there are more than a thousand messages but the one that I want to look at is at the top. I never have to delete these messages, Yahoo takes care of it automatically. The only requirement is that I log in to the service within a set period usually 90 or 120 days to keep the account active.

Once you are comfortable that the site concerned is legitimate, you can then change your email address to your primary one. On the other hand, if you start getting spammed, then you do nothing, let the spam emails build up and get deleted automatically by the system.