If you're using Yahoo! in conjunction with the Mail for Windows 10 app, you might have received an error message stating that the Mail app can’t sync Yahoo! email following updates to the Yahoo! account security settings.
This error refers to the security protocol currently being used by the Mail app. To fix this problem, Microsoft has added OAuth support for Yahoo! accounts in the Windows 10 Creators update. To install the Creators Update now, go to the software download site, and select Update now. For more information about the update, see Windows 10 Creators Update is here.
Note: If you have a Japanese Yahoo! email account you will need to add your Yahoo! account as an IMAP account. To do this, use imap.mail.yahoo.co.jp for your incoming mail server and smtp.mail.yahoo.co.jp for your outgoing mail server, and follow the steps in the Add an account using advanced setup section in Set up email in Mail for Windows 10.
If you're still having problems after installing the update you can enable two-step verification and then create an app password for the Mail app as a workaround. The app password will only be used by the Mail app.
Enable two-step verification and create an app password
Turn on two-step verification in Yahoo! Two-step verification requires the use of a phone number associated with your account.
Create an app password.
In the left navigation pane of the Mail app, right-click your Yahoo! account > Account settings. Enter the app password that you created in step 2.
Alternatively, you could temporarily allow access to the Mail app by changing the security setting on your Yahoo! account. See App access and security for instructions.