cannot add one of three email addresses, using verified address, user name, and PW.

I successfully set up three of my Google Gmail addresses on Thunderbird. I tried to set up my fourth and primary Email address, which is on a Spectrum server, using the absolutely correct address, user name, and PW. Thunderbird will not accept it, saying there is an "authentication error", but not identifying what it is. How is it possible to fix an error, if no precise error is specified? Thanks for any help anybody can offer. Phil (cpliechty@gmail.com)