Go to the Google Play Store and download the free YMail App (Yahoo Mail)
the stock email on the device is buggy and terrible as an email client.
After you get the stand alone app, login with your yahoo name and password. Then click on settings to set notifications sounds, how much mail to get, and when. Its really very easy.
Then go into the stock email program and delete the Yahoo Account. You should not have them both at the same time for the same account.
We noticed also an issue with email body, but in respect to emails send out from "Nook", (what is a Barnes and Noble book reader/tablet, with some other functions and apps.)
My granddaughter has one, and we communicate via email without an issue, but not with on Razr/Droid:
In that case the beginning line of her reply appears only in the email list page. When I open the email itself, I see only a copy of the email to which she typed the reply. Her reply does not appear at all (missing email body). I think that droid's email reader has a problem, its not about using a separate email client for every each email provider.