New Plugin From Yoast Lets You Email Commenters
Check out this handy new plugin from Yoast called Email Commenters. It offers you an easy way to e-mail all the people who commented on a certain post.
For example, if you’ve updated a post with new information or updated a file offered for download within a post, this plugin gives you the ability to notify everyone all at once. Simply install and activate and you’ll get the new icon attached to the posts, only viewable by the admin.