Ok, one more thing: Is the address 10.73.219.156 the only IP address on the external interface of the server1? If you don't bind the 10.73.219.156 IP address to the ethernet interface on server1, then hosts on that network won't be able to find the server even with the prerouting rule. You could solve this by Proxy-arp or just simply adding another IP address to the outside interface. This may be redundant, but I don't believe the external interface's been discussed at all as a possible issue.
