It's just the nature of decentralized + secure + anonymous (at least to the transport layer) ... you can't realistically have all three without being open to effectively a DDOS, poisoning the system as a whole.

I'd worked on a proof of concept that would have all three of the above features using a DHT for delivery, relying on collisions for addressing, so that targets overlap, but a given target could only decrypt messages actually to them... the down side is it would be possible to send MANY messages to a target, so many that the system would be brought to a crawl and effectively useless.