If you've ever wondered whether you're using the correct method to connect with your customers, then check out this post from Seth Godin.
Basically he's saying that you need to know the purpose of your communication before choosing what the customer (or potential customer) will get from you.
Email is a powerful medium (especially opted in email) because its entire purpose is to prompt for a response, to start a conversation or discussion that may end in a sale, but will result in more knowledge on both sides.