I recently completed a new Exchange 2010 installation and a lot of the clients were still running Outlook 2003.
To enable Outlook 2003 connectivity to Exchange 2010 we need to enable encryption between the client and server.
If there are a lot of clients we want to enable this via group policy , however the Office 2003 template does not contain the option to enable encryption.
Microsoft released a group policy template. Outlook_2003-Exchange_RPC_Encryption.adm
So once your group policy object is created do the following, user/computer configuration , expand policies , right click on Administrative Templates and then add/remove templates and then add the downloaded Outlook_2003-Exchange_RPC_Encryption.adm package