How can I select and delete all of my unread Yahoo emails all at once?

There is no other way since you don't want to delete them all. You can have 500 per page on the classic mail, which makes it a little easier. Place a check besides each one you want to delete, then click delete. If it's an entire page, click Check all, then delete. On the Classic mail screen, click Options. Under General go to and click Results per page. Click the small arrow, now you can choose up to 500 per page. Click Save Changes, upper left. You can change this any time you like. You can do this for the Sent folder if you don't need them. Then go back to and open the inbox, scroll to and click Check all, then delete. Don't forget the trash. Using the new mail, click Options or more options on your mail screen. Under General Options page go to Message settings. Check the button, Show all messages in a scrolling list. Click Save Changes, upper left. Now your emails are on one list, same page, so you can delete them all at once. Then click Select all, click Delete.