Event id 9496 MSExchangeIS

exchange2013 CU6运行一段时间后，发现大量的错误日志

Log Name:      Application
Source:        MSExchangeIS
Date:          11/19/2014 11:23:17 AM
Event ID:      9646
Task Category: MAPI
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      sunluen-tx9.sunluen-tx.local
Description:
Mapi session /o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=user1f51f092 with client type MoMT exceeded the maximum of 250 objects of type Message.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="MSExchangeIS" />
    <EventID Qualifiers="49156">9646</EventID>
    <Level>2</Level>
    <Task>6</Task>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="2014-11-19T03:23:17.000000000Z" />
    <EventRecordID>492491</EventRecordID>
    <Channel>Application</Channel>
    <Computer>sunluen-tx9.sunluen-tx.local</Computer>
    <Security />
  </System>
  <EventData>
    <Data>/o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=user1f51f092</Data>
    <Data>MoMT</Data>
    <Data>250</Data>
    <Data>Message</Data>
    <Binary>5B444941475F4354585D00001C000000FF24000000000000000200000000</Binary>
  </EventData>
</Event>

我查了没有user1f51f092这个用户，这个错误是什么意思？要怎样解决？