First of all try closing the app by double tapping the home button and hold down the mail app that will appear at the bottom(scroll side to side if necessary). Press the close button in the top corner of the mail app to close it. Now try delete the item. If not turn the iPad on and off and try again. If not go onto safari and log into your emails there like you would if using a pc and try deleting.
Should've asked are you using in mail app or using safari to access your emails?