The need to email came up when I wanted to notify commentators of my previous announcement posts regarding my plugin - Facebook Like Thumbnail, about its new version which is a rewrite and has lot of goodness in it. I looked it up, and I found a plugin by Yoast, which just adds all the commentators in Bcc and let you email using your mail client. I definitely wanted a better solution where they are emailed separately along with their actual comment so as to remind them that they left a comment on my site earlier and this email is in regard to the same topic.
I build a free plugin, which is now hosted on WordPress.org repo - Email Posts Commentators, and the way it works is, you can select multiple posts to be processed together. Specify your own email as Bcc and even lets you exclude email addresses from the comments of the selected posts. You write down your message along with email subject and an email goes out to them. Take a look at the screenshot to see how its admin screen looks like and how does the email itself look like.
Email that goes out
If you run into issues, using the plugin, please report them here - https://github.com/ashfame/email-posts-commentators/issues