I have a few suggestions for implementing the email notification and reply.
NodeBB already has privilege section in the admin panel. We can add a "receive notification" option to the privileges. This setting will allow the admin to select groups which will be notified by default when activities occur in a category
NodeBB can also provide a notification section in the user settings. The user can specify categories to receive emails; and the different activities to receive notification for.
A user will automatically be notified about activities on a topic when he creates a topic or comments on one. There can also be a checkbox to allow users to unsubscribe