Hey, thanks for reading. I think it's possible to do something like this on Nostr. I'm not that well-versed with the protocol but it looks like you can do it either through the client or the relay.

Adding a small, local cache inside a client could make it more resilient and delay-tolerant. That could help in catching any posts that you might have sent offline and the client can just rebroadcast the contents of its cache once it's back on the network.

I think a more interesting, and bodgy, approach is having a private relay talking to a public-facing one. The public relay can take any messages from a specific npub key and store it until the private relay requests for it and vice versa. At that point, you're basically redoing POP3/SMTP though. It's interesting to think about that though.