It sounds like the inbox.dbx file got corrupted during the compression.
I have never had nor seen this issue before so I did a little research.
I came across a program called DBXtract that is suppose to be able to recover messages from a corrupted .dbx file.
McAfee has a how to use page that you should read first that can be found here:
http://ts.mcafeehelp.com/displaydoc.asp ... goryId=269
But the download does not work from the McAfee page so with a little searching I found this link:
http://www.pchell.com/downloads/DBXtract.zip
Good Luck