connecting Outlook 2003 to an Exchange 2010 server

Using its default settings, Outlook 2003 won't connect to an Exchange 2010 server (nor will Outlook 2007 that has been upgraded from Outlook 2003). Exchange 2010 requires encryption between clients and itself when doing MAPI connections. As such, you need to make that change from the default settings on Outlook 2003. Just check the box for "encrypt data between Microsoft Office Outlook and Microsoft Exchange Server" on the security tab after clicking on the more settings button when setting up an Exchange account.