iMessage does not have a mechanism to verify the devices associated with the destination account is actually theirs. It is feasible to assume an attacker/Apple/NSA could register an additional device key associated with your iMessage ID and snoop all future messages sent to that user from that moment on, even if they are not able to decrypt past messages. (This is true even if you assume iMessage client binary does what Apple says it does and is not tampered with/backdoored).