They can get your email address from a number of different ways. a couple of methods they use are...
- Your email address may be published on your website, and it hasn't been encrypted.
- Your email address may appear somewhere else on the internet, or you have used it in a forum posting
- Spamming robots may have found your website and is then using a dictionary to guess valid email addresses that may be use.
- Someone that you communicate with has your email address in their address book, and their computer is infected with spyware or a virus which is transmitting their address book to the spamming robots.
There are also many other ways spammers can acquire an email address.