Did you create an app password for your account? https://help.yahoo.com/kb/learn-generate-third-party-passwords-sln15241.html

My IP, Frontier.com, is only powered by Yahoo, so when I navigated your link (thanks) all I got was a Frontier page and a phone support number. No account security page. I'll call that number, speak with a Frontier tech, and he or she won't know what I'm talking about. If you know another way to access the security page, which I presume is yahoo, let me know. Otherwise, thanks for the assistance.

The link provided by christ1 opens a yahoo page here, but I'm not on the Frontier network. While it may be necessary to use an app password to connect to a yahoo mail server, I don't see any mention of app passwords on the Frontier site. They recommend pop3.frontier.com for the incoming POP server, port 995, SSL/TLS security, normal password authentication.

i don't know if using these settings will make any difference - perhaps Frontier can advise.

Can you login here with your Frontier address and password?

The error message given by the OP indicates Frontier uses Yahoo, hence the suggestion about Yahoo app passwords.

Thanks for your assistance. I can log into Frontier with the link you gave me, but I get a yahoo sign-in page. Confusing. My TB account settings show pop.mail.yahoo.com as incoming POP. Since I started pursuing this, the Frontier techs have changed the account settings several times from pop3.frontier back to yahoo, etc. I think there's some error in their server configuration for my address and password, but they don't know how to fix it. Anyway, many thanks for getting back. I'll check with Frontier again . This should be good.

I will call a spade a spade. Yahoo is broken, and personally I think they are on the way to becoming yet another failed internet company.

All these ISP's that they provide services to have no idea about mail or the yahoo service. That is why they contracted it to the lowest tender. Yahoo acts in a completely independent way and as ATT's customers have found. makes changes that lock them out of their account that take weeks to resolve. The ATT experience however did make it clear that you could log into yahoo using your ATT password and username. Perhaps the same works for frontier.

Regardless, the Yahoo mail service is plagued by minor outages and strange intermittent errors. Searching on the error and yahoo makes it quite apparent that mail client users such as those of outlook and Thunderbird, but also the gmail web interface have been seeing this type of error since at least 2017. The only consistent part of it is it is internet service providers that contract their mail to Yahoo.

Personally I think it is all around Yahoos determination to totally use oAth2.0 or application passwords with all mail clients, and the total failure of their ISP customers to be aware the system is broken as they do not have oAuth2.0 "secrets" to use nor do they offer an app password generating option. Perhaps you can make the breakthrough to getting the Yahoo debacle fixed.

Thanks Matt. Appreciate your forthrightness. I've heard rumors to the same effect; how it will play out is anyone's guess. In the meantime, I'll keep at it. Not going to give up Thunderbird for one of those flat, one-dimensional webmails. Cheers.

Personally I am recommending to folks to use GMX instead. Basically the largest English language mail provider in Europe.

https://www.gmx.com/ using the collector it will get mail from yahoo. you get a new email address but all your old email collected and it is a large provider with more than 20 milion customers. https://www.gmx.com/company/about/#.9041400-footer-nav1-1