Cisco Firewall :: 5510 - Can’t Access External IP From Within LAN

Basically we have different customers using the same 5510 firewall. We have created one sub interface for every customer on the inside interface. There are differed NAT rules for every customer all using the same block of public IP addresses on the outside interface. They do not have access to each other’s network so I cannot make any exemption rules between two sub interfaces. The problem is for all our customers that they cannot communicate with each other over Internet, Email, Applications etc. using the external IP address. A work around is to use a proxy server, but they do not agree with that. I cannot make exemption rules between sub interfaces for security reasons.