In one implementation, a computer-implemented method, includes receiving, at a computer system, a message for delivery to a client computing device; determining, by the computer system, a unique identifier for the message; generating an identified message that includes the message and at least a portion of the unique identifier; and sending, by the computer system and to the client computing device, the identified message over a first communication channel and a second communication channel, wherein the first communication channel is different from the second communication channel.
