Problems with Downloading iPhone emails
If your iPhone displays "this email has no content"...
The problem can occur when
using IMAP and the user opens the message before it has been fully downloaded or
read from the mail server.
This is a bug that appeared in iOS 7 and still exists in iOS8.1.
Even if you refresh/check for new messages it has already flagged those with 'no content' as previously downloaded, so it doesn't check or download them again.
Resetting and restarting the App may trigger it to try and download these messages again but this doesn't always work.
Changing the email from IMAP to POP usually fixes the problem.