I have added your information to our open engineering ticket. Updates will be posted here in Community.
Have you tried removing and re-adding the account? Does the account work normally on the Yahoo Mail app?
i have trying adding and removing atleast 20 times so far with no improvements.
Sometimes it works fine for weeks and other time it happens 2-3 times in a day.
It does seems to work fone on web and yahoo app.
Can you tell me how are you accessing your email on the iPhone? Is it Via the Native mail app, the Yahoo Mail app or another 3rd party app?