When multiple external IP aliases are not working, this is likely to be an ARP cache issue.
If the alias IPs you have defined, see our help topic, Adding new interface connections, cannot be pinged from outside, or configured port forwards are not working, it is possibly due to a stale ARP cache issue.
Try resetting the ARP cache on the router in front of the Smoothwall firewall on some routers this may require a reboot.
Also make sure that the IP addresses used, are being routed to your Smoothwall, and that no other machines are using the same IP addresses.