I tried turning SSL off on outgoing mail but it didn't work until I also changed the port from 465 back to 25. Something very odd is going on at ATT Yahoo!
By the way, I'm sporadically getting the same error message on incoming mail but that is not consistent. I get it maybe 1 our of 4 receive attempts.
From my experience, ATT Yahoo takes forever to fix problems so I'm gonna stick with this "solution" until it stops working and then change it back and see if that works again.
Very strange! (But boy! What a relief to see this thread posted. I was pulling my hair out since this started yesterday afternoon.

)