Cannot Add RODC to Exchange 2010

Hello,

I am trying a testing scenario where I installed a Read Only Domain Controller on the DMZ and put the whole exchange 2010 there.

I tried to change the (Modify Configuration Domain Controller) in Exchange to the RODC, but the problem is that Exchange will not give RODC as an option for Domain Controllers. It still needs to access the primary DC. I want to disable the access to internal network completely from Outside and from dmz too.

Please advice.

Regards,