When you track an email, Mailbutler attaches an invisible pixel to your message. That pixel is tracked - meaning we will let you know whenever it is accessed. But because all your recipients receive the same email, they also receive the same pixel - this is why per recipient tracking is not available with the current technology.
Multiple recipient tracking is something we'd love to offer in Mailbutler and we will definitely be working on adding this in the future.
However, if you're using Mailbutler Business, you have the option to view Tracking Details (when, where and what device your email has been opened) which should give you a better insight to how each of your recipients is reading your email.
For more information on tracking be sure to check out this article.