Utility Room

Here's a standard view on Mail.app (OS X 10.5).

If the selected message on your message listing (showing header info, number 1 in the image) does not show up below (number 2), but instead Mail.app shows an another message, your index may have gone bad.

This is nothing to be afraid of. To fix the problem, close Mail.app, (as always, make necessary backups), trash a file called Envelope Index in /Users/YOU/Library/Mail/, and restart Mail.app (there will be a dialogue to import your account, fear not, and click Continue). You might even save same disk space and speed up your Mail in the process! Full details (and more hints) on macosxhints.com.

Keywords: Mail.app, corrupted index