messages stopped working on my iMac computer I have an old iMac running High Sierra. I use the Messages app often throughout the day. Today I received my new iPhone 11 Pro. My phone is set up and seems to be working fine. But on my iMac, my messages quit working if sending to a non-iPhone account.  I don't know if the 2 are related, it could just be a coincidence that messages stopped working when setting up my new iPhone. I have stopped and restarted Messages and I've restarted my computer. Both to no avail. 

Does anyone know how to troubleshoot/fix Message delivery on macOS?
The registration can fail and need to be re-established. Before you contact Apple support, I would open messages on the Mac and sign out of your AppleID. 


Sign out of messages with your AppleID
Check the date / time in the Mac in system preferences. Within 60 seconds of reality should be fine, closer is better of course, but it’s not sub second critical. 
Sign back in to Messages (quit the app and retry after signing out if it’s stubborn). https://support.apple.com/en-us/HT204408


The system should log an error to the diagnostics or the screen if your sign in fails.