Replies via email have been just enabled in this pilot site.
This should enable you to reply to topics via email notifications. Once this feature is tested and satisfactory, we can move onto new steps:
- Creating a new topic via email.
- Emulating mailing list behavior via Discourse.
The current setup is temporary (using a plain GMail account) but it should be good enough to test and learn here, while preparing a proper plan for production.
Related Phabricator task: https://phabricator.wikimedia.org/T184592