replyPUSH

Much like WordPress.com’s “Comment Reply via Email”, you can reply to any post you subscribe to.
Integrates with the wonderful <a href="https://wordpress.org/plugins/subscribe2/" title="subscribe2" rel="ugc">subscribe2</a> and <a href="http://replypush.com" title="replyPUSH.com" rel="nofollow ugc">replyPUSH.com</a> service to provide the email reply magic.
It automatically adds notifications for comment posts to keep the conversation flowing 🙂
All the subscriber need do, is reply at the top of their quoted email notification.

Reply By Email

Allows your users to reply to articles / blog posts and comments via email.

Jennifer M. Dodd

bbPress Threaded Replies is based on comment-display functions from WordPress 
and topic-display functions from bbPress. Settings can be adjusted on the bbPress 
Forums options page of WordPress. bbPress running as a WordPress plugin is required.
bbPress Threaded Replies is incompatible with the Fancy Editor; enabling threading will 
automatically disable the Fancy Editor.
The plugin checks first in the stylesheet and template directories before reverting 
to the default plugin templates.
<h3>Credits</h3>
Development funded, in part, by Ariel Meadow Stallings and the Offbeat Empire.

bbPress Threaded Replies

Add threaded (nested) reply functionality to bbPress.

Reply By Email vs bbPress Threaded Replies