If you have applied SP6 to your BES you should install Maintenance Release 5. Ensure all BES updates are installed whilst logged on using the BlackBerry Service Account (e.g. BESadmin). To see if cached mode on Outlook is causing an issue log onto your mailbox using OWA and mark a few items read (this will quickly show if Outlook is the cause).
http://www.blackberryforums.com.au/forums/blackberry-announcements/4428-bes-4-1-service-pack-6-mr-5-exchange.html
