Dear James Doig
Thank you for posting to Microsoft Community
Based on your description, I understand that you are not able to configure your Spectrum mailbox account in New Outlook, please point out if my understanding is wrong.
Recently many users in the community using the same domain are also experiencing the same issue(Unable to log in/unable to synchronise emails). The main domains affected are Specturm domain and comcast domain. Microsoft has now taken note of this issue, please be patient and wait for the issue to be fixed in the subsequent update of New outlook.
In the meantime you can try to access your Specturm domain mail using other mail applications such as Thunderbird.
This posting will remain open for other thoughtful users to express their views. I will also get back to you here as I learn more about the latest situation.
I wish you all the best
Carmid.L-MSFT | Microsoft Community Support Specialist