I built nostrdb specifically to support the outbox model (which is now fully integrated into ios, android, and desktop), since i wanted the client to only talk to a trusted local relay, as connecting to random relays can cause DoS/verification issues.

It's better to just dump data in from remote relays without trusting if the results actually match the filter or not.

We don't support it yet, but it should be pretty soon. We are building a DM app in notedeck. This is where outbox is really important so that you don't sent giftwraps to public relays, only the users inbox relays.