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There’s a new iMessage for Android app — and it actually works
(www.theverge.com)
This is a most excellent place for technology news and articles.
Notifications are generated after the message is pulled from Apple's servers to your device.
The push messages just tell your device a message is available to be pulled. It doesn't contain any message contents or metadata.
It's basically just "you have a new message waiting" and then your phone will ping Apple's server to request the message.