It is not that difficult as we think to delete all emails in yahoo inbox at a time ;). I will now explain how to delete all emails in yahoo inbox at once.
After a year, I logged in one of my yahoo inbox and for my surprise there was more than 11000 unread emails.
I was shocked and was worried how to delete all the emails, except few important emails at the last.
So first I created a new folder and moved all the important emails to that folder.
Then now comes the trick
Switch to new yahoo mailbox and check all mails( see highlighted red square)
now click on delete .. hurray I deleted all yahoo mails more than 11k in one go :)
