Here's how to mass delete more than 50 emails. Go to your Yahoo Inbox. DO NOT click the Select All box! Instead start scrolling down to the bottom of your emails. ↓ Once you get to the bottom you may have to wait a second for more emails to load. Keep Scrolling Down! Keep Scrolling! Once you've gotten tired of scrolling down, then Click the Select All Box! Then, click Delete! You will see a pop up box asking if you would like to delete , for example, 2000 emails! The number of emails to be deleted will depend on how far down your list of emails you scrolled before clicking the select all box.
Although this is also tedious, it is better than just deleting 50 emails at a time. I hope this helps. 😉