Delay in receiving outlook messages with exchange 2000

i have an exchange server 2000 and outlook 2000 and 2003 for the clients . The problem is with receiving outlook message in all user ,sometimes the message can be received after several hours. we had to do sometimes a restart to the exchange server , and lately we had to do it every day , even after the restart we have to wait maybe half an hour to get the messages and for everything to work fine. there are no error messages in the event viewr of the exchane server . i don’t know what causes this problem and of course do not know how to fix it.

thans