If possible go to the webmail site and delete it there.
Restart your iPad.
If it still shows up go into Settings > Mail, Contacts, Calendars and open the accounts details. If it has an ON/OFF switch for mail in that account, turn it off. Open Mail (this forces the app to clear out all the accounts contents from Mail). Go back to settings and turn Mail back on.
If there is not ON/OFF switch, then delete the account. As before, open Mail to force it clear. Go back and re-add the account in Settings.
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When you have problems with an app it's a good idea to clear it from RAM.
Close/Clear the app: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app. This will remove it from RAM and insure a clean start the next time you launch the app.
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears. Apple recommends you restart the iPad at least once a month.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart. takes a bit longer than a restart.