To creat a great email you need to communicate to your customer. Communication is you sending a message and them recieving it. They will only recieve a message if its of interest to them. Better still, it gives them value. So when you communicate with your customers giv ethem value. Make the message all about them.
A great email will communicate things of benefit or value to your customers. Two simple approaches are to offer them a product with great value or offer advice that is of great benefit.
This sounds simple, so why do most emails just talk about you and not the value to the customer?