There is no other marketing tool as effective as a list of emails people have voluntarily given you. Besides the fact that it is proven out by every metric tested, it just makes sense: people gave you their email, so they are receptive to your pitch. They may even be anticipating it. Email lists have the greatest success rate of any other “advertising” and their cost-per-click? Absolutely free until you have at least 500 subscribers, at which point it is profitable to pay for the mailings you are doing.