If the sender has received earlier a private message from the receiver
it should have cached the Client ID from the SILC Packet Header.
+If server receives a private message packet which includes invalid
+destionation Client ID the server MUST send SILC_COMMAND_IDENTIFY
+command reply packet destined to the client with error status.
+
See [SILC2] for description of private message encryption and decryption
process.