Outlook cannot connect until I run an ipconfig /release then /renew

Some users regularly experiment the following : for some reasons, their outlook cannot connect to the Exchange server (2003 SP2) at computer startup. I tried to ping the server and it responded. Even if I restart Outlook I still cannot connect. What I have to do is an ipconfig /release then /renew and restart Outlook and it is then OK.
Any idea what the problem might be and how to solve this ?