Yahoo Mail New (Yahoomail) offers unlimited storages; you can save every email you receive in Yahoo!The new feature “Time-saving email experience” of Yahoo Mail gives user to peek inside any message with the reading pane. Or open it up and quickly toggle back and forth between messages. Users can chat each others and text directly from their inbox.
However, some users are having a lot of strange problems with the all new Yahoo Mail currently. One solution is to switch back to Yahoo Mail Classic. Just switch back until the problems are resolved.
The big problem here is how can you switch to Yahoo Mail classic? There is no link for you to click on the Yahoo Mail Classic while you are loading your all new Yahoo Mail.

Try this link to switch back to Classic:
http://us.f504.mail.yahoo.com/dc/troubleLoading

Or refer to here

If the problem still unsolved try, the following method.

For Internet explorer users,
Select Tools > Internet Options.
then choose delete temporary files, cookies ,browser history etc.
Then close explorer then open again.

For Firefox users,
select Tools > Clear Private Data.
then choose your ticks and Clear Private Data Now.
Then close Firefox then open again.

Else, you can ask to get help from here.