First, please double-check to make sure that the e-mail server settings in Thunderbird for that account are what Yahoo says they should be:

From: https://help.yahoo.com/kb/pop-access-settings-instructions-yahoo-mail-sln4724.html

Incoming Mail (POP) Server
   Server - pop.mail.yahoo.com
   Port - 995
   Requires SSL - Yes

Outgoing Mail (SMTP) Server
   Server - smtp.mail.yahoo.com
   Port - 465 or 587
   Requires SSL - Yes
   Requires TLS - Yes (if available)
   Requires authentication - Yes

Your login info
   Email address - Your full email address (name@domain.com.)
   Password - Your account's password.
   Requires authentication - Yes

In Thunderbird, Thunderbird menu: Tools: Account Settings (If you cannot see the Thunderbird menu, press the Alt key or F10 key on your keyboard to show the menu)

For incoming e-mail: Select "Server Settings" on the left side under your e-mail account, and the settings are on the right side.

For outgoing e-mail: Select "Outgoing Server (SMTP)" on the left side, then select the server you want on the right side. Click the "Edit" button to change the settings.

If you already have the correct settings, then this might be a glitch for that account in Thunderbird.

Try adding the same e-mail account again to Thunderbird. Thunderbird will complain that two account have the same name, and prompt you to change the name of the new one.

If the new account works better, then move all the e-mail from under the faulty account in Thunderbird to the new one. Then remove the faulty account.

If you need/want instructions for how to do this, please ask.

Thanks for your reply. Yes, settings in Thunderbird are as you describe and have been for years. As I stated, I have another Yahoo Mail account with the exact same settings in Thunderbird and Yahoo and it works just fine. The problems with the account I am writing about began a week ago or so, I just can’t sign into Yahoo from Thunderbird; although even with that error message which I showed in my post, I’m able to send mail; strange. If I left Thunderbird open on my desktop while browsing, I would now and then receive the mail, which is in Yahoo as Thunderbird checks for new mail every 10 minutes. Have not been able to sign into my Yahoo pop account from Thunderbird since before yesterday nor get the mail, which is in there. I don’t think it makes any difference, but perhaps I should state that both the working and non-working pop accounts with Yahoo are free pop accounts through Yahoo Singapore, have been for years and working just fine. Will try adding the same account again in Thunderbird, and report back.

This person has the same problem with Yahoo POP: https://support.mozilla.org/en-US/questions/1155165

So possibly the problem is with Yahoo.

If creating the account again in Thunderbird works, please let me know ASAP, so I can instruct this other person how to do it.

Am working on it right now, seems to work OK, got over 300 saved messages in sub-folders to transfer after which I'll test it one more time, will get back in about 20 minutes.

Seems to work; did see that message pop up one time; the same "new account is now an imap account, will switch back to pop and start all over again moving those saved e-mails,

Am having a tough time creating a new account for pop3.; after entering all the correct information I keep getting "configuration could not be verified - is the user name or password wrong?"; can you help?

You said that you have "Two-step verification disabled and Allow apps that use less secure sign-in enabled".

Perhaps Yahoo is not cooperating in this respect. They really don't want people to use the less secure sign-in.

So try enabling two-step verification, and create an app password to use in Thunderbird. Yahoo: Manage app passwords Understand and create 3rd party app passwords

IMPORTANT

I also use POP with my Yahoo e-mail account, and now it keeps complaining that I have the wrong password.

Thus there must be a problem with Yahoo POP e-mail servers right now.

Will look at your link later, right now Thunderbird won't let me create a pop3 account, on the "re-test" it tells me configuration was found, on the "done" it tells me that my password or user name is incorrect even though I use the same as in my imap settings and which also are in password manager, I think if there were a problem with Yahoo my other account with them wouldn't work either.

Just sent a message from within Thunderbird from the new imap account to the other Yahoo account and vice versa; both messages were received just fine. I'm just unable to convert the new imap account to pop3 in Thunderbird. "password or user name is incorrect."

In the Yahoo Help Community forum, other people are also saying they are getting login fails for POP.

I logged in to the other yahoo pop account just fine and received my sent message from the new imap account. I'll keep working on it and will be back, right now my problem is I can not set up a pop3 account in Thunderbird because Thunderbird tells me my user name or password is incorrect; which they are not. Please, let me know if you ascertain anything else on the subject.

I changed my password for my Yahoo account, and now I have no problems with Yahoo POP.

Can't change password because I can't set up a pop account in Thunderbird.

I mean change the Yahoo account password using your web browser: https://login.yahoo.com/account/security

That did it! Deleted the new imap account I created in Thunderbird today, removed all passwords for that account in password manager, went to yahoo mail and changed my password, created a new pop3 account in Thunderbird with the new password and everything is back to normal. Still don't understand why my other pop3 account with Yahoo was not affected by this. Sure hope it stays that way. Thanks for sticking with me and for all your help.

jedakay said

That did it! Deleted the new imap account I created in Thunderbird today, removed all passwords for that account in password manager, went to yahoo mail and changed my password, created a new pop3 account in Thunderbird with the new password and everything is back to normal. Still don't understand why my other pop3 account with Yahoo was not affected by this. Sure hope it stays that way. Thanks for sticking with me and for all your help.

Yahoo are forcing password changes on all users affected by the 1 billion user data breach. Basically like everything they do these days it appears to be mishandled. Perhaps get a real mail provider that actually understands security and email. GMX is good, as is, dare I say it, AOL.

Changed password on the problem account back in June when the security issue within Yahoo first trickled through. Why would Yahoo insist that a second password change is necessary? Why was my other Yahoo pop3 account not affected by Yahoo's dubious method of doing things? Albeit, I use that account very little. In any case, the simple password change did the trick, all still seems to be working this morning.

Perhaps I celebrated too early. Am now getting the same error message on the other pop3 account with Yahoo on an intermittent basis. Just send a test mail to that account and this time it came through OK in Thunderbird. Probably will have to change the password for that account also.

jedakay said

Changed password on the problem account back in June when the security issue within Yahoo first trickled through. Why would Yahoo insist that a second password change is necessary?

See: Yahoo Security Notice December 14, 2016 Also: Mail Client Failing to connect to Yahoo Using POP?

Like Matt says, this is a matter of Yahoo not doing things properly in many respects:

1. Security breach that they didn't know about, and took a long time to fix when they were notified of it.

2. Sending out notices to some Yahoo users, but not others, that they needed to change their password.

3. Suddenly disallowing POP logins for lots of people, if they don't change their Yahoo password, with no explanation.

4. When they knew there was going to be problems because of #3, they post on their forum instructions (Mail Client Failing to connect to Yahoo Using POP?), that have nothing to do with the actual issue.

The fact that people using Yahoo IMAP e-mail are not having any problems indicates to me that Yahoo doesn't want people to use the older POP protocol. If they say that, though, they will incur the wrath of millions of people who still want to use POP. Instead, they would rather make things so difficult for people to use POP, that they switch to IMAP on their own.