Does this help? https://support.mozilla.org/en-US/questions/1446501#answer-1651121

Yes it did. The mistake was not using entire email address as username

Thanks

I closed Thunderbird and re-opened and now I can't connect to the Yahoo SMTP server again.

check that the setting you changed was saved.... It is an ongoing problem with some antivirus products that setting changes are not saved because the antivirus blocks the change. Disabling the antivirus is a short term response. Creating an exception in your antivirus product for the Thunderbird profile folders is a better response.

Duplicated https://support.mozilla.org/en-US/questions/1507695