This plugin (available here) is for sites that are accustomed to using Discourse as an email list and who, like in the 1990s, be able to email a response directly to the user.
This is not really recommended unless your community fits this very special niche, as it leaks user email addresses via emailed notifications.
I do not intend to provide free support for this plugin, but if you have questions or comments please feel free to ask them here.