I've become accustomed to using imessage on my macbook, and at one point about a month ago i signed out, and when i signed back in it no longer worked. It would not deliver my messages and kept telling me the user "is not registered with imessage" when i know for a fact they are because i use imessage from my phone to talk to these people. Yesterday I tried to sign in again and it just so happened to work. Today however, I had to sign out while I took an online quiz so I would not be distracted, and upon trying to sign back in just an hour or so later, I'm facing the same issue again. Any suggestions to get it to work? I've rebooted my computer, and I've signed out of imessage, as well as just tried to disable and enable it again. Nothing is working, it's so frustrating! Thanks in advance!
Open Messages(iMessage) and go to the preferences. Go to the accounts tab and click on the account you use. Press the "-" at the bottom left. Then press the "+" at the bottom left. Enter your account information and close the preferences. Now try using Messages(iMessage). If it still doesn't work, than you might want to take it to an apple store.