Here is an iPhone Email Tip that will hopefully save you some frustration:

When reading your email with your iPhone, have you ever got the following message?

"This message cannot be displayed because of the way it is formatted. Ask the sender to send it again using a different format or email program. Multipart/Alternative"

If this happens to you, you can work around the problem by first deleting your message. Then go find the message in your trash and move the message back to your in-box. You should now be able to read the message properly.

Apparently this was an issue with iOS 3. It was fixed in iOS 4. Unfortunately the bug seems to be back again in iOS 5.