That case was one of the strangest we bumped into.
After a major power failure, we were unable to open items in inbox. It came up with an error such "can’t open item".
Apparently if you have Norton Symantec 10 and on (Endpoint 11 etc) the problem might be a key in registry or a file that was damaged during the power cut.
What you need to do is removing the Antivirus and installing it again in the exchange server where the Symantc antivirus is installed.
Pay attention: not restart services or stop Symantec antivirus service – a complete uninstall and install.