I have to say that, having dealt with this over and over and over for the past few weeks, it's a mystery. I have alternately set-up my devices (MacOS/Sonoma on a Mac Studio and and a MacBook Pro, and iOS on an iPhone and iPad) using both the Yahoo route (using my Yahoo/AT&T password) and the Other/Mail Account route (using my Yahoo Security Key), and from time to time I still get connection errors — regardless of which route I take — or if I do maintain a connection, the retrieval of email is very sluggish. In my last effort a few days ago, the Apple Senior Advisor escalated the issue to the engineers — I was informed that a software fix, from the Apple end, should be in the works. In the meantime, following another round of failed connections, I rebuilt all four devices using the Yahoo route. After that, the next day, this morning, both my MacBook Pro and iPad failed the connection test. I didn't rush to "delete and rebuild" — instead I waited it out, shutting down Mail, shutting down my MBP, shutting down my iPad…waiting; and eventually — as I said — a mystery, the connections re-established, however, still rather slow on the draw. I suspect that the Yahoo servers may be at fault; but I'm going to let the Apple engineers deal with the Yahoo people, because if I call AT&T support, I will just be told to delete the account and rebuild it. Not again. So now all my devices are working, using the Yahoo route, the AT&T/Yahoo password, and I'll let that ride.