Email not delivered until a message is highlighted

Hi,
We have a Exchange 5.5 sp4 with some critical hot fixes/patches applied. We encounter this problem in the past three weeks. Some users said that not until they click something (i.e. highlight an existing email message in Outlook), no message will deliver to their inbox. The email messages that I am talking about are internal email. This problem doesn’t seem to happen to all email users. And it doesn’t seem to matter what Outlook version it is.

Please give me some pointers as to what cause this. Your help is fully appreciated.

Wing