May be it was end of the last year, I was assigned to finish a wordpress project which was taking way too long time to deliver. I was away from wordpress development for more than two years. So after jumping into the project I found most of the user requirements are bit different than the general.
One of the requirements was to wordpress admin should be able to use “admin comment” section to reply the comments as email whether the user is subscribed to the post/comment or not. If some one comment into the content and if admin approved it with reply, the user will have to receive an email with the reply.
Worpdress provide a nice action call “comment_post“, I wrote a simple method which will be executed after every comment and check is it approved or not and will send an email. Check the full plugin from github
By the way, without subscriptions into the post and comment, sending email is not “ethical” so if you want to use it use in your own risk.
Also feel free to fork it if you want to add/customize