multiple exchange servers in same site

I have configured a third exchange service for testing purposes and would like to have have it work with my other Exchange servers. They are all under the same Domain, two of the Exchange Servers are in the same group, but the thrid is a stand aloneServer. How do I get the third Server to send E-mail across to my other Exchange Servers? I have tried a site connector but it gives me an error when I configure it.

Thanks,