denishua

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/

Comment Reply Notification

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.

Hamid Reza Yazdani

If you have a blog where users have questions with repeated answers in the comments, or even if you have an online store and typing or copying/pasting repeated answers takes a lot of time, this plugin will help you get useful and beautiful answers to your users in the shortest possible time. 
After installing and activating the plugin, just go to the “Frequently Replies” section under the comments menu and define your replies. From now on, by selecting any reply, add it to your user’s comment answer with one click.
<h4>Contribute and translate</h4>
Head to the <a href="https://github.com/hamidrezayazdani/WP-Frequently-Replies?utm_medium=referral&utm_source=wordpress.org&utm_campaign=wp_org_repo_listing" rel="nofollow ugc">WP Frequently Replies GitHub Repository</a> to find out how you can pitch in.
WP Frequently Replies is translated into multiple languages, including English and Persian. Help localize WP Frequently Replies even further by adding your locale – visit <a href="https://translate.wordpress.org/projects/wp-plugins/wp-frequently-replies/?utm_medium=referral&utm_source=wordpress.org&utm_campaign=wp_org_repo_listing" rel="nofollow ugc">translate.wordpress.org</a>.

WP Frequently Replies

If you are tired of copying/pasting duplicate responses to your user's comments, this plugin is for you

Comment Reply Notification vs WP Frequently Replies