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I updated to Yosemite a while back, but yesterday iMessage started sending the error message "Not Delivered". I can send messages to non-Apple devices (SMS?) just fine, but not to Apple clients (Apple server?). No problem at all with iPad Air or iPhone 5S, both iOS 8.1.3. Any suggestions would be greatly appreciated!
-Macbook Pro Retina (late 2013) -iPhone 5S -iPad Air