Successful Email Marketing is 1-to-1 Permission Marketing
Email's strength is personalization. Instead of the same boilerplate message going to all of your house list at the same time, truly successful email marketing consists of personalized messages being sent to customers individually.
These messages take into account the customer's interests, what she bought in the past, how often she buys on average, her budget, etc. This is one-to-one marketing.
Tailoring your communication to the needs of the individual customer is only possible if you know a lot about her. In order to find out all this, and to enter and continue the dialog at all, you need her consent. The recipient gets only the marketing messages she is interested in, and you get her attention and, if you get it right, eventually her money. This is permission marketing.