How do I stop the Yahoo sign-in request pop-up on my Outlook 365 Inbox?

I would first make sure the accounts are removed from Outlook before you delete the OST files from the IMAP accounts. Click File, Account Settings, Account Settings, and remove those that are redundant.

You might wait some time before deleting the OST files to make sure you were not using them. If you were using them, you can add the accounts back in Outlook (File, Add Account).

The OSTs are here, C:\Users<account>\AppData\Local\Microsoft\Outlook.

The PSTs are here C:\Users<account>\Documents\Outlook Files

I originally had pop3 att email settings. When the popup started in early August, I could not use Outlook until I logged into Yahoo. None of my passwords worked for it, so I had to open a new Yahoo account. That changed all my settings to yahoo servers, killed my incoming email, and changed the account type to imap.

Then I re-set up the pop3 account with att servers to be able to synch again with my iPhone via iTunes. After running .pst repair programs several times, got the incoming mail to work again.

Now I have multiple data files and Outlook preview shows multiple email accounts and I still get regular yahoo popups insisting that I log in.

The pop3 database file is in one drive and it appears the imap file is in another location.

Since I want the pop3 to be the default, can I now delete the imap database file?

By default when you add a Yahoo/AT&T account to Outlook it is added as IMAP with Yahoo server settings. If you manually added the account as POP3 and you used the AT&T server settings then you need to use a Secure Mail Key/App Password.

If you are seeing the Yahoo OAuth prompt though I suspect you have an IMAP account with the Yahoo server settings. If so, you need to use the regular password and not the App Password.

To double check here are the server settings for both:

POP3 and IMAP Email Settings - AT&T Email Customer Support (att.com)

IMAP server settings for Yahoo Mail | Yahoo Help - SLN4075

To check from Outlook do the following:

1. Click in the Inbox of the AT&T account
2. Click File, Account Settings, Server Settings

To check if its POP or IMAP

1. Click in the Inbox of the AT&T account
2. Click File, Account Settings, Account Settings
3. Look at the Type next to the account

This is for a pop3 email account.

If you see the Yahoo OAuth prompt you need to use your regular password which is the same that you use to sign into webmail, How to add a Yahoo IMAP account to Outlook Desktop without an App Password - Microsoft Support.

If that does not work, please check if you have Outlook In App support and submit a ticket (Help, Contact Support).