Note : If you are using yahoo mail classic then follow these steps to delete all of
your inbox mail with two clicks. This may also work for yahoo mail plus, I am
not sure, but you can try it. Yahoo is currently
using two graphical user interface for Yahoo Mail. The Yahoo Mail Classic
and the new version “All-New Mail“. If you want to delete all messages
from Yahoo Mail inbox, you must switch to the new version of
Yahoo Mail.
Step 1 : In Yahoo Mail Classic somewhere in the right side is a link that says “try the new yahoo! mail” Click on it to switch to the new yahoo mail .
Step 2 : Click on the check mail and when the page loads click the very top check box under the word delete and that will select all of your inbox emails.
Step 3 : Then click the Delete button and all of your emails in the inbox will be deleted.