Yes, it is, but they have that same feature on a big central server now.
Discussion
Maybe time isnt required, but order (sequence).
Nostr signing to outbox could include a sequence kv (tag, number) - e.g. public:665, dm: 34323. You declare in outbox who can fetch your current sequences counts.
Would let clients find dropped posts, and let users know what they have signed.
What about just an incremental nonce per npub (like in web3)?