Found it, it was nostr:npub149p5act9a5qm9p47elp8w8h3wpwn2d7s2xecw2ygnrxqp4wgsklq9g722q , now need to find his post
Discussion
This is the post, I am not sure if Niel thought about making it interoperable with existing DMs but I liked the design and it was in my mind since then to make an email like interface for NIP-17 DMs
nostr:nevent1qqs0le2nayu8gnetmn0nng356wt7ctaa0wpk9ly7dxesxsqcn4kp33g40e3qm