It maps on best to the "Mail" content type best. That's what I'm after.

Nostr Article → very clear what the content type does

Nostr Video → same

Nostr Repo → same

Nostr Mail → same

Nostr Message → Uhm, wait what? What are we talking about.

Nostr DMs → Oops, that cannot be a thing without a centralized server.

Also:

Nostr Private Group → very clear that it's something like Signal/Simplex (not Nip-17)