I just tried sending email to ZX12R@freerepublic.com and it was returned. So, I think it is just internal mail.
Vanity E-mail
As a valued supporter of Free Republic, we have reserved nonvalueadded@freerepublic.com for your use. E-mail sent to this address will be forwarded to your Contact E-mail address.
Current Value: Disabled
It’s under your Account settings on your profile. I’m not sure if it’s still only for contributors or not.