Outlook Web Access Problem

Our company moved to Microsoft 2003 to 2007 servers. The Microsoft WEB access mail is continuously asking the users to put in user name and password. On every mail you click the server first request username and password. If you go to a different folder you are requested user name and password.

The users experiencing this problem is going through a proxy and firewall of the client site.

Users in the same environment do not have the problem. Any ideas out there?