So this plugin apparently hasn't been updated in a while, but I have it on my blog and it works well. It's easy to install and use. When someone leaves a comment on your blog, if that comment is replied to the original commenter will get a short email telling them there's been a reply to their comment. You can customize the email (subject and body) as well, but I think the default settings are fine. Not everyone who comments on your website will check back to see if someone's replied to their comment, so this plugin comes in handy.
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