Sorry for my bad english, it's not my native language. I had the same problem. I tried EVERYTHING, even restore the cellphone and contact Apple Support, and they couldn't solve it. I don't want to use the Yahoo Mail app, because it doesn't synchronizes calendar and reminders. So, I tried something that's working for me so far. Go to settings, tap mail, contacts, calendars, go to get data. In Yahoo, you should have activate the push function. Tap there. You'll see three options: push, get data, and manual. Change it to "get data". Go back and choose how often you want to get data: 15 minutes, 30 or an hour (don't put it in manual). And that's it. You will not receive mail notifications immediatly, but al least you'll have them within the next fifteen minutes. I hope this help somebody. God bless you!