A site user should receive email notification about every new comment added by another user. Here I will tell how to setup such notifications.

Solutions:

Use Subscriptions Module

• Download Subscriptions
• Description:
  This module enables users to subscribe to be notified of changes to nodes or taxonomies, such as new comments in specific forums, or additions to some category of blog. Once enabled, all nodes will have an additional link that allows the user to change their subscriptions. Users have tab on their user screen to manage their own subscriptions. Users can also set an auto subscribe function which notifies the user if anyone comments on posts they have made. Admins can set this on by default.

Use CommentMail Module

• Download CommentMail
• Description 5.x-0.1 (2007-Jul-25): module sends email notification to admin about new comment . Links in this email  allows quickly approving, removing or blocking IP-address of comment’s author.

Use Notify Module

• Download Notify
• Description: Sends info about any published material to email (sometimes with text body). Only for registered users. Developers advise to install 5.x-1.x-dev instead of 5.x-1.1. Notification about new comment sent to author of an article missing.

Setup:

• Set a special parameter to that user won’t receive notifications after a certain number of fails.
• If links within notifications don’t contain domain name (http:///./user/1/notify), insert the following to settings.php:
  

  $base_url = ‘http://example.com'; // NO trailing slash!

• Replace comment.module file with a file that is provided together with the module (comment.module.dp5) for the module correct working.

Use Comment Notify Module

• Download Comment Notify Module
• Description: module is updated more frequently than notify. Works as for anonymous as well for registered users. Read Install.txt for more details.

Features:

• notifies anonymous about new comments
• notifies registered users about new
• allows users to cancel notifications delivery in one click
• allows registered users to set their notifications on their profile

Summary

• Comment Mail is the best for admins (notifications & management)
• Comment Notify is good for all users (both registered and unregistered)
• Subscriptions is good to notify author of an article about new comments and track new comments on a site.