Yahoo! Mail starting to implement OAUTH2
 

I couldn't find the thread on this topic. I do remember this being a discussion and that Yahoo! Mail set a deadline implementation date in which all third party email clients would cease working with Yahoo! Mail unless the client supports OAUTH2. That date came and went, nothing happened. Fast forward to now.

I have three Yahoo! Mail accounts, two (POP3) are mine, the third (IMAP) is organizational. About a week ago I was not able to log in via my 3rd party email client (Claws-Mail) to my primary Yahoo! Mail account. I was able to log in via web interface. Today this happened with my organizational account. One account left to go I guess.

Here is the fix: Generate and manage third-party app passwords

FYI: Claws Mail currently does not support OAUTH2, there is a patch available for the current version (3.17.8). I am using a beta version 3.99.0 which is now support GTK3 but not OAUTH2.