Like many, I use Messages on my iMac, MBP, iPhone and iPad to text non-Apple phone users. Recently I got a new iMac and set up the new machine with backups from my old iMac. I have been using Messages seemingly without issue, but realized a few days ago that I an unable to sent out messages to SMS users and the account on this machine hasn't been syncing the messages I have been sending from my other 3 devices. The other three all send, receive and sync completely. The new iMac ONLY does these things with other Apple users. No SMS sending, receiving or sync. When I try to send out a Message on this machine, it immediately says "Not Delivered" with the red exclamation point. When I click on the name of the intended recipient, it lists the phone number along with the following message: XXX-XXX-XXXX is not registered with iMessage. You can turn SMS relay on from your iPhone. I have been to my iPhone and I believe I have this activated. As I said, it works flawlessly on the iPhone, iPad and a MBP. I have turned the machine on and off, closed and re-opened Messages and even logged out and back into the Messages account. Anyone ever experienced this? Thoughts on a fix?