For a pure client, outbox is good, it's a big improvement. But what servers can do is completely different. Outbox can tell you where something is located so you can try to get it JIT. That doesn't mean it won't be slow or even work at all. Servers can try all day long to get it, even while you're phone is powered off. All it needs is a moment in the day where the relay is working, so it's available for you instantly when you go online.