As I understand it, Keyhotee does the same thing as Bitmessage in that every mail you send out is sent to every node in the network, and whoever has the corresponding private key can read it.
I cant think of any other way for a decentralized system, but is there really no alternative to this? It doesnt seem to be very sustainable with millions of users, especially on mobile devices which cant afford to constantly sieve through millions of messages.
Btw perhaps this issue should be part of an FAQ pinned on this forum.