Exchange won't play
 

We have Laptops out in the field that connect to our system over a VPN link.
However some of the laptops will go from working fine one day to not being
able to connect to the Exchange server the next.

The server is running Exchange 2003 and the laptops are running Outlook
2007. The user can see the mapped drives on the system and can ping the
Exchange server but Outlook can't connect to Exchange.

What could be causing the problem?