Unable To Delete Emails From IPhone 6
                     Unable To Delete Emails 
From IPhone 6
If you are unable to delete Emails from 
iPhone 6, follow the below-mentioned 
troubleshooting methods. A list of Solutions 
mentioned below makes use of it.
Step 1 :
On your IPhone 6 main screen, tap the Settings icon.
Step 2 :
Select the Mail option and tap your email account from the displayed list.
Step 3 :
Select the Advanced tab followed by Deleted Mailbox.
Step 4 :
Under On IPhone, touch the Trash option.
Step 5 :
Go back to the Accounts window without closing the Settings window or 
canceling the changes in the settings.
Step 6 :
Tap the Done button to save the changes.
More Info: https://mail-setup.com/unable-to-delete-emails-from-iphone-6