1) Call it "Mail", not "DMs"
2) Have sealed an unsealed. Marketplaces do indeed need signatures for proof.
Your app can than just look at relevant "Mails", without asking Chat apps to mash things like this together in one Chat convos feed.
Chat and Mail are not the same concept. Communication channel VS One-off messages.
Nip-17 suits Mail better.
Some (if not most) Mails need signatures.
