E-mail delays in inbox

[Danny]

We are using Outlook 2000, Win2000 Professional system
and Exchange Server 5.5. For some unknown reasons, few
of our Outlook 2000 clients won't see the inbox mails
after they open the outlook until they click on one of the
inbox messages, then the incoming mails flash into the
boxes. We don't know what could cause this "clog".

Also when they send mails out, the mails have been
delivered to the destination, but their Outboxes show
Outbox(1) until they click on it. It shows "Outbox"

Please help
Thank you,
Danny