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?