Im having problems sending messages from my MacBook to a friends MacBook. I keep getting told that my friends email address isnt registered with iMessages. Ive selected this friend from my contacts (within iMessage) and seeing as he hasnt got an iPhone Im selecting his email address. Im happily sending messages from my macbook to other friends with iPhones. Could it be that this particular friend hasn't set up iMessages on his MacBook? Or perhaps hes registered a different email addess with his Apple ID? We are both running brand new MacBook Pros with Mountain Lion.