This is showing up in our event log and after some searching, I have no idea how to troubleshoot this. I tried various searches on Google but the best I found was that it might be a mailbox with over 100,000 deleted items or something. and I did find two boxes where is looks like folks never "empty" their deleted items but I am not sure that is a fix for this this issue.
Log Name: Application Source: MSExchange Mid-Tier Storage Date: 12/6/2012 1:02:03 PM Event ID: 5001 Task Category: CopyOnWrite Level: Warning Keywords: Classic User: N/A Computer: server.local Description: Multiple errors occurred while saving items for mailbox ''. The error was encountered 51 times on this session with the client information: . Event Xml: <Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event"> <System> <Provider Name="MSExchange Mid-Tier Storage" /> <EventID Qualifiers="32768">5001</EventID> <Level>3</Level> <Task>5</Task> <Keywords>0x80000000000000</Keywords> <TimeCreated SystemTime="2012-12-06T21:02:03.000000000Z" /> <EventRecordID>1133716</EventRecordID> <Channel>Application</Channel> <Computer>server.local</Computer> <Security /> </System> <EventData> <Data> </Data> <Data>51</Data> <Data> </Data> </EventData> </Event>