One can permanently delete emails from your yahoo account.Need to follow below-mentioned steps:
1.login to your Yahoo account
2.Find "Trash" option on left side of you yahoo account.Then click on the trash and select Ok.
It will permanently delete your emails reside in trash box.
You can also contact to their customer service number 1-855-777-5686.For any further issues you can visit this link yahooservices.edublogs.org/2017/01/23/why-messages-sent-from