Unfortunately, in this digital age, spam is everywhere and almost impossible to avoid. Using major players like hotmail, gmail, yahoo can help since they have decent spam filtering. However, good email will once in a while make its way in there as well. Now where do the spammers find your email address? Well there are a multitude of ways that range of someone infected with malware/spyware that has you in their contacts (address book) to making your email address public via a blog, forum, etc;
Here is a link to a GREAT FREE TOOL that will do all that for you at a click of the mouse.
DESCRIPTION FROM THE AUTHORS WEBSITE: