Apple has removed Trash All functionality from Mail app since iOS 10. So in iOS11 for many users deleting and selecting all email together to delete them all together is not a easy find. Deleting all/multiple email is required by many user to free up space in iPhone memory or remove some unwanted emails. Even the problem is reported by many new iPhone X and iPhone 8 user when they are not able to delete email from their iPhone.
How to delete/remove all/multiple emails on iPhone 8/iPhone X in iOS 11:
1. Open the inbox and then tap on “Edit” at top right cornor
2. Tap on any message on screen so the checkbox appears next to it.
3. Now press and hold the “Move” button with one finger, and while holding the “Move” button, uncheck the message you previously checked
4. Now release the “Move” button.
5. You’ll now be at the Inbox selection screen, at the top of the screen will show how many emails are selected, choose the “Trash” icon to move them all to trash and permanently delete them.
Now your should be able to move them to trash at once and delete easily. I hope it helps to resolve the issue in iPhone.