When you upgrade your organization to Exchange 2010, your clients running Outlook 2007 or later versions will automatically be compatible with the change to RPC Client Access, since they support RPC encryption by default.

Outlook 2003 doesn’t use RPC encryption, however, and RPC Client Access requires it by default. If you haven't turned off RPC encryption, your users will need to configure Outlook 2003 for RPC encryption or you'll need to use a Group Policy to force Outlook 2003 to use RPC encryption.

To configure Outlook 2003 to use RPC encryption, use the following steps.

  1. Click Tools > E-Mail Accounts > View or Change an Existing Account.
  2. Select the account and click More Settings.
  3. Select the Security tab.
  4. Select Encrypt data between Microsoft Office Outlook and Microsoft Exchange Server.
  5. Click OK

Or you can disable RPC encryption on the CAS Servers, off course I do not recommend this but it is doable.

Set-RpcClientAccess –Server <ServerIdParameter> –EncryptionRequired $false

To verify the RPC encryption status at you server, simply perform:

Get-RPCClientAccess –Server <ServerIdParameter>

One more thing, for outlook 2003 compatibility with Exchange 2010 you have to consider that UDP notification support was removed from Exchange 2010. As a result, Outlook 2003 can only use polling notifications in online mode. This will result in a slight delay in updates to item status (30 seconds on average with up to a one-minute delay) when changes are made to items in a mailbox accessed by Outlook 2003. There are two workarounds for this issue:

· Use Outlook 2003 in Cached Exchange Mode.

· Adjust the polling interval on the Client Access server. This will impact the performance of the Client Access server.