When you are sending emails, make sure you know who you are sending them to. If you have a business, this is a great way to keep in touch with your customers and clients. You know what they want. You know how they feel. You know how they act. So, target your emails to meet those wants, needs, and actions. Maybe it is a promotional email with a sale (or a few). Maybe it is a new product or service. Maybe it is an update or new information. Whatever it is, they already know you so they will be more likely to open it and keep it.
On top of that, you must have a clear knowledge of the person you are trying to sell your products to, so you have to clearly define their personality, lifestyle, interests, hobbies, desires, financial situation, family situation, health questions, etc. And you have to be able to see it! You have to have that clear picture of your ideal customer in your head, so you can write a message that will resonate with them.