You didn't give any info about your RHEL samba configuration or your Windows domain, and it'll be hard for anyone to help you without that.
Assuming your Windows clients are in a Windows Active Directory domain, I think your samba server on RHEL needs to be a member of the windows domain in order to accept logins in the domain\username format. That is, you'll need to specify SECURITY = ADS, and authentication will actually be done by your Windows domain controller.
I've never worked with a samba server in a Windows domain environment, so I doubt I can help you much, but I think you'll need a kerberos realm, openLDAP, and winbind setup on RHEL to do that. From what I can see, it's not trivial to set it all up.
Last edited by Z038; 05-12-2013 at 05:46 PM.