Default outlook 2003 cannot connect to a default exchange 2010 instalaltion. This is because exchange 2010 uses by default RPC encryption to communicate. Outlook 2003 has this default disabled.
Solution: Enable encryption in outlook 2003.
You can do this at,
then on the tab security,
enable “Encrypt data between Microsoft Office Outlook and Microsoft Exchange Server.”
Other options to solve this problem is disable rpc encryption on the Client Access Server (this is not the prefered option) or via outlook profile or via group policy. All options are described in this knowledge base article.