I too started having issues yesterday. I tried yesterday morning to turn on 2 step authentication for my account and now have to wait 3 days before I can authorize it. Since then, imessage has been haywire on my mac and my 6 plus.
One will receive messages and the other won't and vice versa of that. For some contacts, outgoing messages will simply send. For other contacts, the progress bar at the top moves almost all the way to right and it just gets stuck on "sending". It never even bumps it to the top of the queue of my message chains. For yet other contacts, it goes through but as a green message and not a blue imessage.
It's very frustrating but I'm hoping it's just due to the icloud 3 day wait thing.