The domain controller functionality which Exchange 2010 expects can only be provided by an Active Directory Domain Controller. RHEL as it is cannot provide it with the Samba version that is available on RHEL. Please note that Exchange 2010 server expects more than just Kerberos KDC from a domain controller.
Samba project implements AD DC-like functionality starting with Samba 4.0. There are some details as to implementation; in particular, Samba 4.x cannot yet to be compiled with MIT Kerberos KDC (available on RHEL) to provide AD DC-like setup. The version of samba4 packages in RHEL 6.x is only suitable for using with FreeIPA identity management solution (which cannot yet be used to host Exchange server).
The only other solution would be to compile and configure Samba 4.x by yourself, using embedded Heimdal Kerberos KDC, as described http://wiki.samba.org/index.php/Samba_AD_DC_HOWTO. However, this configuration would fall outside of a supported RHEL setup, at least for my understanding.