Automatic account configuration doesn't work for all accounts out there. In such cases, manual account configuration is the only way. Use that manual config option to enter the settings given to you by Spectrum.

Thanks Stans. You are right; automatic configuration didn't work. I manually configured the servers and it still doesn't work. I suspect there may be something strange going on with the password so I'll try to get the password changed (not a simple task) and try it again.

I changed my password for TWC/Spectrum email and it fixed the problem. I don't know why, but it works now. The old password worked through the web link but not through Tunderbird. Go figure.