How to set the logon domain in Outlook 2010?

Hi all,

in Outlook 2003 it was possible to set the logon domain via Regkey:

[HKEY_CURRENT_USER\Software\Microsoft\Exchange]
"LogonDomain"="mySecondDomain"

In Outlook 2010 this does not work anymore. How can I set the "mySecondDomain" in local Outlook installations? This is necessary because we use a second domain which is not operated by us. Therefore we cannot use "myFirstDomain\firstname.lastname" for the logon to the Exchange servers.

Can someone please help?