The problem started before the new Yahoo mail was released. I thought it was caused by the Android 4.3 update. But a Samsung rep installed 4.4.2 and the phone still does not sync. We've gone thru the phone exhaustively, I've deleted and reinstalled Yahoo mail accounts at least 6 times, and no one can find the problem. I wrote to Yahoo and of course they insist it is not their problem since my mail accounts sync just fine on my computer and my tablet. On my phone, syncing happens randomly - maybe 4 emails sync show they were read on another device, then the rest of the day no emails sync at all. I started using AquaMail that someone in this forum suggested, and that works just fine for both Yahoo business and Yahoo personal accounts. But the native Samsung mail app is useless right now for my phone.