I had a random problem come up. A user who has been using Outlook Deskotop software (relatively recent, I think Outlook 2019), to access Yahoo via IMAP and SMTP.
It had worked fine for months and months until yesterday. The errors came with the codes 0x800CCC0f and 0x800cc0e.
For some reason the first results in my Google searches didn’t get me the fix – it took way longer than it should have to find out that Yahoo, in late 2020 started phasing in mandatory app passwords. See below for how to generate an app password in the Yahoo web interface, and then pop that into your IMAP or whatever email client.
It’s a good change, I assume it means that a compromised plain text password on one device can be “voided” without having to update passwords on other devices.