I saw a post similar to this recently. To not allow Thunderbird would be to not allow any third-party access. My assumption is that Spectrum just doesn't like providing assistance to users who use email clients instead of the web interface.. If your email is not working, please post screenshots of incoming server setting and SMTP server setting, and someone here may be able to assist.

David, Thank you for the timely reply! You were on the money! It seems that Spectrum (Charter Communications) no longer supports third party clients. All went South when I opted for Fidium fiberoptic internet, but I forgot about hardware limitations like my 5 year old computer that won't support that speed. When I went back to Spectrum (the only cable offering in my area), the back connection was no longer there because of non-support. If I hadn't opted for the change, all would have been fine!