delayed email delivery for laptop users

I’m running Exchange 5.5 sp4 on a nt 4.0 server sp6. I’m using outlook 2000 on my laptops running nt4.0 sp6a. Numerous users don’t get email into there inbox unless they read a message or move a message into another folder. After reading or moving there here the sound stating mail has been delivered.This problem only occurs on laptop users not desktops users. There could be messages from two hours earlier. Mail is not being sent with a delay. Any Ideas?