IMO it's a feature for a separate plugin - not this one. This plugin, as the description suggests, is using Notification API, and not Push API.
A Push API-based notification plugin would be great, but it's not just an upgrade to this one as Push API works in fundamentally different way by using a service worker.
I setup nodebb-plugin-emailer-smtp-complete in order to send my email to a local postfix relay but it fails when you do not use auth.
I created an issue on the plugin's github, but have noreal idea how to fix or I would make a pull request.