Emails are being sent from past posts

We use Feedburner and RS Feedburner (previously Feedsmith) to distribute emails of new posts to those who are subscribed to our blog. The problem that we are having is that emails are being sent from blogs that have been previously published (even from years past). How can we correct this issue? http://blog.interface.com/

https://wordpress.org/plugins/feedburner-email-subscription/