Exchange 5.5 Application Error log. MSExchange IMAP4

Hello, Experts. As I look through our Exchange Server 5.5 stand.'s events log, I found this log from Application event,

Event Type:      Error
Event Source:      MSExchange IMAP4 Interface
Event Category:      Authentication
Event ID:      13004
Date:            2/28/2007
Time:            4:19:33 PM
User:            N/A
Computer:      LAEXCHANGE
Description:
Logon attempt from 10.1.3.95 has failed: AcceptSecurityContext() call failed with error Access denied.

It records every 10 minutes basis on Exchange Server's event viewer, but the user (10.1.3.95) has no problem to send or receive emails at all.
And I can't find any informations to troubleshoot except article KB220905 but that article was for different case. The client (10.1.3.95) using Macintosh OSX 10.4.6 and Entourage for his Mail application. We have other users whose using Macintosh Mail program (IMAP) but haven't seen any errors except that one.
Any help will be appreciated. Thank you.