Thanks for the write up. I have been trying to get this to work for the last 5 hours. Could only find FAQ's, WIKI's and Forums that were old and using outdated settings and settings that are no longer available in the software or they are using Outlook, a few months old, or people saying "Same as me" or "Me too", which really doesn't help the issue. I then searched for the newest and found this entry. Made the changes (OAuth2), even brought that up to Yahoo support, and I was able to send emails.
When I reopened Thunderbird a rather big pop-up box from Yahoo opened and wanted verification that I wanted to use a 3rd Party email client and asked for my U/N and P/W.
One thing that is not brought up, that the Yahoo support had me try was the 3rd Party Email client or application password creation on Yahoo's security page. It creates a password for your application other that Yahoo's app, and support had me enter that password at one time also. I don't know if that has/had any effect on this issue.
I made the mistake of "testing" the transfer with my primary account email, and when you do that it automatically transfers any email addresses set up under that account.
THANK YOU for your write up. It most definitely helped this poor slob.
