Since I use Outlook 2019/Office 365

Let me understand this.  You rather forward your att email to some other email service rather than change all your devices email client apps to use OAuth or alternatively a secure mail key if your client doesn't support OAuth(2)?

Just get into the att/yahoo webmail and set up a forwarding address for that account.  Click the settings gear -> more settings -> mailboxes -> click account under Mailbox list.  A pane will appear on the right from which you can set up a forwarding address (under Forwarding).  Part of the setup will include verification of the destination address but sending an email to the forwarded address which has to be acknowledged there to complete the verification.  After verification all email sent your att/yahoo email address will be forwarded to the forwarding address (except stuff filtered as spam) will be forwarded to the other address.  A copy of each forwarded email will also be saved in your att/yahoo inbox (or wherever it's filtered into).  And you still have to periodically visit the att/yahoo email to check for false positives - stuff mistakenly considered as spam.

By the way, I don't believe you can set up forwarding from an email client app.  It's too late by the time the client sees it.  The POP/IMAP server actually does the forwarding which is why you have to set this up from the webmail.