Do you have the authentication method set to OAuth2 for the outgoing (and also the incoming imap.mail.yahoo.com or pop.mail.yahoo.com)? Remove the entries for the account from Saved Passwords in Settings, restart TB, enter the account (not app) password in the OAuth window to allow access, and an oauth:// token will be stored in Saved Passwords (cookies must be accepted in TB Settings).

Thank you for your prompt response, sfhowes! I appreciate it. You asked me a question in the first sentence of your advice about what I have the authentication method set to. Hopefully, I can find the answer in Thunderbird's settings. I will take a shot at following your advice! Cyd

The Yahoo Rep may well have been correct as many Thunderbird users have antivirus programs that are actively preventing. or passively blocking through their incessant scanning, the saving of changes to the profs.js file where your settings are. Folk never notice until some major change it not saved.

Do you have an exception in your antivirus for the Thunderbird profile folder?

Here's an update to my initial question: After receiving sfhowes' response, I contacted Cox Total Care and provided his response to my problem. The rep graciously agreed to help. He changed one of the settings to "OAuth2" as directed. I think that may be all he did, and the problem has been resolved! Thank you so much! Matt- I just read your reply. I do not know the answer to your question. But for now, I am going to not address it, since I think the problem has been resolved. If the problem pops up again, I will ask a professional to look at the issue that you raised. Thank you for taking the time to assist. This is a great community, and I am glad that you so graciously assist those of us who are far less technologically proficient! Cyd