I think the Front - End exchange server is in the DMZ and then I suppose that you need to have clients and other exchange server communicating to this Front- End device so I don't know that is the Front - End exchange server ip address, and my assumption is that the clients are in the inside.
What ever you need to do this is what we need to keep on mind:
1. If you want to have communication between the inside and the DMZ device is to have dynamic nat ex.
nat (inside) 1 IP 255.255.255.0 0 0
global (DMZ) 1 interface and permit the traffic in the outbound and dmz-in access-list
2. If you want to have access to have traffic from outside to DMZ we need to have a static translation static (inside,outside) public ip address internal ip address netmask 255.255.255.255
and permit traffic in the inbound access-list