Problems connecting Outlook 2003 to Exchange 2010 could turn out to be an unpleasant surprise after migrating to Exchange Server 2010 over the weekend. The problem is caused by Outlook 2003 not using encrypted RPC connections to the Exchange Server by default, and Exchange 2010 requiring  encrypted RPC connections (contrary to earlier Exchange versions). The solution is simple but you have several options; The way you should proceed not only depends on your situation but you also need to check the company’s security policies regarding communications encryption which might restrict your options.

Change how Outlook connects

Enabling RPC encryption in Outlook can be performed per configuration (Outlook profile) or using a Group Policy Object.To manual change the way Outlook connects:

1. Open Control Panel > Mail > Show Profile > <Select Profile>
2. Select Properties > E-mail Accounts > View or Change existing e-mail accounts
3. Select Next > Microsoft Exchange Server > Change > More Settings > Microsoft Exchange Server > Security
4. There, check Encrypt data between Microsoft Office Outlook and Microsoft Exchange Server
5. Close everything with OK > Next > Finish > Close > OK.

You can also control the RPC encryption setting centrally for Outlook clients using the following registry value as part of a GPO:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\11.0\Outlook\RPC
DWORD: EnableRPCEncryption
Value: 1

For a more detaild guide on implementing the Outlook profile change or implementing the GPO using an administrative template, consult KB2006508.

Change how Exchange 2010 accepts
To change the way Exchange 2010 accepts RPC connections, i.e. disable the RPC encryption requirement, you need to disable the RPC encryption for Exchange Server 2010 CAS servers (remember, in Exchange 2010 RPC connections are handled by the CAS server role), use the following cmdlet:

Set-RpcClientAccess –Server <Server Name> –EncryptionRequired $False