Mailing lists on nostr, if they really want to mimic mailing lists (which I think they should do better, but if), can use the inbox model and messages deposited into people's inboxes just like email does, with subject tags. And some client manages the mailing list, you message it to sign up, etc.

But as I said, I think forums on nostr can be better than email mailing lists and shouldn't do that. They should be more like communities instead.