Why do people open emails from companies? What makes an effective marketing email? Well these stats from Marketing Charts seem to show that deals are the primary reason that emails effect buying behavior.
But this also covers info regarding other forms of marketing. Email ranks at the top of this list and I think it might have something to do about permission marketing. You have to opt in to receive those emails which means you are already interested in the product. If you are the marketer you have already qualified the prospect. Permission marketing is the way to go!