It is a virtual domain add on which piggy backs on top of the original virtual domain currently in SME 5.
What it allows you to do is forward mail and web traffic as it is queried to your servers named/DNS settings, this reduces the load on your server but is invisible to internal users.
With both virtual domain and advanced virtual domain I created the domain name needed.
Then in ProxyPass I setup the Internal IP address needed to direct the traffic to other box.
Don't ask me why it did not work with the regular virtual domain and ProxyPass?
No idea.
Soon as I added the adv. virtual domain RPM, it worked like a charm.
Must have activated the nessesary files which are present in server/gateway conifguration, but not in server only configuration.
Mark