When you delete a message from your Inbox, it is sent to a "Trash" folder for that account. If you would like to permanently delete the messages in this folder, take the following steps:
1. Tap on the menu (hamburger icon) on the top left and scroll down to Accounts.
2. Tap on the account you would like to permanently delete messages from. You will see the list of folders for that account.
3. Scroll down and tap on the "Trash" folder.
4. Within this folder you can swipe left to permanently delete a message, or tap and hold to select multiple messages at once.