When a reply is made to a comment the user has left on the blog, an e-mail shall be sent to the user to notify him of the reply. This will allow the users to follow up the comment and expand the conversation if desired.
Detail information:
http://theme10.com/comment-reply-notification/
中文介绍：
http://fairyfish.net/2008/11/03/comment-reply-notification/
1.4 2010-10-13 tk (by thomas) : the e-mails are now sent even when the reply is posted from dashboard
1.2 2009-02-18 fix the problem with WP 2.7 paged comments.
1.0 2008-12-19 inition version