Multiple exchange servers in one domain

We have two exchange servers in one domain. We installed the second one and moved all the email from the original to the second server. We decided it was nice to have a fall back exchange server on the network so we did not uninstall exchange. Both servers are domain controllers.
The problem we have found is that if the original exchange server is offline, the second does not seem to work or even let outlook connect to it.
Any ideas are most welcome.

Thanks