Event ID:      539

I receive following logs on my Exchange 2003 server very frequently. Even user' s mailbox is on another server.
Event Type:      Failure Audit
Event Source:      Security
Event Category:      Logon/Logoff
Event ID:      539
Date:            7/16/2008
Time:            7:32:12 PM
User:            NT AUTHORITY\SYSTEM
Computer:      Exchange-Server1
Description:
Logon Failure:
       Reason:            Account locked out
       User Name:      X@xxxxx.x
       Domain:      
       Logon Type:      3
       Logon Process:      Advapi  
       Authentication Package:      Negotiate
       Workstation Name:      Exchange-Server1
       Caller User Name:      Exchange-Server1$
       Caller Domain:      xxxxx
       Caller Logon ID:      (0x0,0x3E7)
       Caller Process ID: 4068
       Transited Services: -
       Source Network Address:      -
       Source Port:      -


For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

AND ALSO following error for same user

Event Type:      Failure Audit
Event Source:      Security
Event Category:      Account Logon
Event ID:      680
Date:            7/16/2008
Time:            7:32:12 PM
User:            NT AUTHORITY\SYSTEM
Computer:      ISB-MSC-NFEXS
Description:
Logon attempt by:      MICROSOFT_AUTHENTICATION_PACKAGE_V1_0
 Logon account:      X@xxxxx.x
 Source Workstation:      Exchange-Server1
 Error Code:      0xC0000064


For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

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