Ok. I know you said you deleted and readded it, have you tried it lately? I ended up having to do that to both of my accounts.
You could also try exiting your mail app. You do that by tapping the home key twice, your apps will come up in a band at the bottom, hold down on the mail app to make the red 'close' circle come up. Tap on that then tap on the home key again to exit.
You can then reopen your mail app and see if that helps.
Unfortunately, there's no singular magic bullet fix.