User being logged out immediately after login

this morning I have one user, out of 110, that can not access his exchange mailbox. Receives the "Folder cannot be opened" error message when attempting to open Outlook.

When viewing the Security log on our Exchange Server, it shows that it is logging in the user and then immediately logging him out. See below. Notice that the time for the logon and logoff are identical.

AND, it never shows him as being logged into the Mailbox Store.

Any ideas what might be going on? Or even better, suggestions on how to correct it?

Security Log:
Event Type:      Success Audit
Event Source:      Security
Event Category:      Logon/Logoff
Event ID:      540
Date:            10/9/2008
Time:            10:24:53 AM
User:            DM576886\UserX
Computer:      MAILSERV
Description:
Successful Network Logon:
       User Name:      UserX
       Domain:            DM576886
       Logon ID:            (0x0,0x63F3F14)
       Logon Type:      3
       Logon Process:      Kerberos
       Authentication Package:      Kerberos
       Workstation Name:      
       Logon GUID:      {c3f84d29-700d-201e-d567-cb8b7c4e0f6d}
       Caller User Name:      -
       Caller Domain:      -
       Caller Logon ID:      -
       Caller Process ID: -
       Transited Services: -
       Source Network Address:      10.204.40.28
       Source Port:      4013

Event Type:      Success Audit
Event Source:      Security
Event Category:      Logon/Logoff
Event ID:      538
Date:            10/9/2008
Time:            10:24:53 AM
User:            DM576886\UserX
Computer:      MAILSERV
Description:
User Logoff:
       User Name:      UserX
       Domain:            DM576886
       Logon ID:            (0x0,0x63F3F14)
       Logon Type:      3