First, close the email app. Double click the home button and bring up the multi-tasking tray. Find the mail app, hold it until it wiggles and tap on the red minus sign to close it. Tap the home button again to bring back the home screen. Reset the device, holding the sleep/wake and home buttons until you see the Apple logo and then release. After the phone restarts, check mail again.
Try terminating and restarting Mail:
1. Press the Home button
2. Double click the Home button to bring up the "Recent Apps" tray.
3. Touch and hold on the Mail icon in the "Recent Apps" tray until a "-" appears.
4. Touch the "-" to terminate Mail.
5. Mail Safari and see if this corrects the problem.
If this doesn't try restarting or resetting your iPhone:
Restart: Press On/Off button until the Slide to Power Off slider appears, select Slide to Power Off and, after the iPhone shuts down, then press the On/Off button until the Apple logo appears.
Reset: Press the Home and On/Off buttons at the same time and hold them until the Apple logo appears (about 10 seconds).