Here's a useful primer from Vertical Response about your email newsletter list.
Bottom line: Don't send unsolicited email, you'll just wind up getting your business in trouble. You might want to try either renting a postal list of potential prospects or looking at a list of leads you might have acquired over the years. Then send them a postcard. In the postcard you can direct them to the web and give them a great offer or a free download so they'll move into the first two lists.
Read the entire article here.