Step1: You open the mail app. 

Step2: You hold two fingers and drag, it multi selects. 

Step3: Keep both fingers in screen. And use your other hand to scroll the emails as fast as you can. 

Step4: Mark unread. 

 Damn. Maybe just delete the whole app? 

 Maybe. But old emails saved my life multiple times. 

Why delete things off of free storage? 

 That’s a damn good point lol