I haven't checked Bitmessage's protocals, but that it is not nessasary to identify the intended recipient. If you publish the ciphertext with no meta-data, then everyone can attempt to decrypt it. Only the intended recipient would succeed, but an ease dropper would not be able to identify who successfully decrypted the message. This raises an obvious scalability question, as you need to attempt to decrypt every message that is used in the network (in addition to simply downloading it which you have to do anyway). There are probably ways to mitigate this overhead without compromising security.