In addition to tracking and reporting on click throughs and open rates, some emails (especially those that come from specific individuals) are likely to inspire not a click but rather a simple reply to the email. At this point, there is no way to track the number of people who only replied to the email, though this is an important "action metric" as well. I would argue replying to and writing a personal email in response to an eblast is actually a higher level of engagement then clicking a link. 

For example, we sent an invitation out to our donors for a recognition event that came from our VP of Philanthropy. I know people replied to the email because we've gotten person messages about the event but I also know they did not actually click anything in the body of the email. This provides less than a full picture of the actions taken from our communication. 

Thanks for considering!