There are many reasons why email is such an effective marketing channel. For one, email is customizable. You can easily tailor your emails to a buyer persona. Also, email is focused because it gives you a chance to speak directly to your contact at a time that is convenient for them. Lastly, email is action oriented. You can use it to direct traffic to your website and ultimately drive sales. It is important to know how your email subscribers are interacting with your email campaigns. There are five metrics that should be considered when looking at your email marketing.
Check out this great article: http://www.business2community.com/email-marketing/email-marketing-5-metrics-measure-effectiveness-0797291#!x2VSA