?!

Discourse is a "mailing list with the interface". It allows both replies over email and starting a new topic over email.

Discourse is a more like a forum with a lip-service email interface tacked on, similar to GitHub or any of the web-generation tools.

At least at some point of time it was possible to order Discourse discussions as emails, which you can reply to, and it worked pretty much like a mailing list. Don't know how it is currently, though.