I came across this piece of information which I believe you should be very aware of during your Exchange 2010 deployment.
During the deployment you will most likely also create a RPC Client Access Array so that your Outlook clients will all be pointing to your Client Access Servers (CAS). This RPC Client Access array is the RPC Endpoint that your Outlook clients will use to connect to their mailbox which, unlike previous versions of exchange, means that your Outlook client will no longer make direct RPC connections to the mailbox server (with the exception of Public Folders) but to the Client Access Servers instead.
However by default on Exchange 2010 the RPC Client Access array has "encryption required" enabled which is fine for Outlook 2007 clients and upwards as they have the RPC Encryption enabled by default but that's not the case for Outlook 2003. Outlook 2003 in this condition will fail to connect to the RPC Client Access service and remain disconnected.
To resolve this problem you have the following options:
- Disable RPC Encryption on the RPC Client Access array.
Set-RpcClientAccess –Server Exchange_server_name –EncryptionRequired $False
- Deploy a GPO that contains the Outlook 2003 settings to enable RPC Encryption
You can start by disabling the "Encryption Required" on the RPC Client Acces Array and then validate your GPO for Outlook 2003. Once it's fully deployed and enabled for all outlook 2003 clients you can enable "Encryption Required" again.
For more information have a look here: