I am currently running Exchange 2003, Outlook 2007
It seem like my exchange server had an unexpected shutdown over the weekend.
When I login to Outlook with Cache Mode on, I am not able to received any email, it doesn't seem to be updateing.
When I login to Outlook with Chache mode off, i am able to see the email in my inbox but when I click on it said "Cannot open this item".
This is happen to my entire orgainization.
If OWA is enabled in your organization, just browse to http://exchangeservername/owa or https://exchangeservername/owa from any of your computers, login there and see if you can open the items in OWA.
This is to rule out Outlook as the cause of the problem.
If that doesn't work too, is there some kind of antivirus software installed like Symantec or Trendmicro which force every mail to go through them before being presented to the clients?