DiscourseEmailIncludeAddress is a plugin for communities that really, really like mailing lists and want to be able to email people when they get a notification rather than replying through Discourse.
It exposes users' email addresses, so I don't recommend it unless your users all expect a level of privacy associated with 1990s era Mailing lists.
Follow Install a Plugin
how-to from the official Discourse Meta, using git clone https://github.com/pfaffman/discourse-email-include-address.git
as the plugin command.
Is at your own risk.
If you have issues or suggestions for the plugin, please bring them up on support.literatecomputing.com.