Connection problem with Outlook 2003

Running Exchange 5.5 SP4 and several versions of Outlook (not Outlook Express) in corporate mode. The Exchange server also is our Internet gateway. We have no problems surfing the ‘Net on any of our computers. The computers running Outlook 2003 have trouble maintaining a connection to the Exchange server, for email, shared folders and scheduling. They do not have problems surfing the ‘Net, only connecting to Exchange. We do not use OWA.

TIA,
Don