MSExchange Mid-Tier Storage

Any one know what this error means?

Log Name:      Application
Source:        MSExchange Mid-Tier Storage
Date:          12/13/2012 11:15:04 AM
Event ID:      5001
Task Category: CopyOnWrite
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      EXCH.CONTOSO.ORG
Description:
Multiple errors occurred while saving items for mailbox ''. The error was encountered 369 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="2012-12-13T10:15:04.000000000Z" />
    <EventRecordID>131218</EventRecordID>
    <Channel>Application</Channel>
    <Computer>EXCH.CONTOSO.ORG</Computer>
    <Security />
  </System>
  <EventData>
    <Data>
    </Data>
    <Data>369</Data>
    <Data>
    </Data>
  </EventData>
</Event>