Comment Notifier lets user to subscribe the comment thread of each single post of your blog. It improves traffic and loyalty.
Notification emails are fully customizable (both plain text or html) with a number of “tags” to insert author name, post link and title, receiver name and so on.
The “subscribe me” checkbox in the comment form area can be added automatically or manually.
Read more on Comment Notifier offical page.
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This plugin is available for download for your site.