It may help if you post your incoming server settings completely and your SMTP server settings. Thanks.

I also have a problem using Thunderbird with Spectrum / Charter. I contacted Spectrum support. Here is what I was told:

"Thank you so much for your patience! I was doing some research and Spectrum is not allowing links with thunderbird server as of March 2024. You can use spectrum email on apple mail or outlook going forward but thunderbird email linking has been eliminated."

The difficulty with the message you received is that there is no 'Thunderbird server' to link to. There appears to be some confusion on whatever Spectrum is communicating .