Sometimes iPhone Mail running on iOS 4 has the nasty habit of displaying blank email messages or loading empty message previews in the Inbox. This problem is easily fixed.
1. By default your message previews should be enabled. To make sure they are on, touch Settings -> Mail, Contacts, Calendars -> Preview and select the number of preview lines desired.
2. If you're running iOS 4 on an iPhone 3G, reset by holding down the home and sleep buttons until the Apple logo comes up on the screen.
3. For iPhone 4 and iPhone 3GS users, double-click the home button to bring up multitasking.