With the built in Messages app, you only can send messages to Apple devices.I don't know the reason, but I cannot get iMessage to send to anything but a, iPhone or iPad. Recently it is messing up if the iPhone is not 'live' and connected to the network. My iPad is wifi only. iMessage is set up with my email address. Maybe that is why I cannot send to a non-Apple device.
With the built in Messages app, you only can send messages to Apple devices.