It’s a well documented issue with Outlook 2003 connecting to Exchange 2010 that means Outlook 2003 is not as responsive in Online mode as it was with legacy versions of Exchange Server (http://support.microsoft.com/kb/2009942).
What is less well documented is an odd error message that can appear because of this interaction.
Imagine the following scenario. User on Outlook 2003 has lots of messages to delete, and deletes them one at a time. Outlook will not refresh the display for up to 5 seconds (the lowest setting that you can tell Outlook to refresh, via the Maximum Polling Frequency registry key). The problem is that if the user deletes a message and it does not disappear from the screen and then (thinking its gone, and the highlight has moved onto the next message) presses delete again. Outlook generates “Unknown Error” – which is not exactly helpful, and could appear as often as every other message that is deleted.
How to fix: Cached mode (though in the scenario I came across the above it was Outlook on a Terminal Server, so that’s not an option), upgrade the client version of Outlook, or use Shift or CTRL select and delete all your emails in one go!