Configuring postfix for virtual host using specific smtp (gmail)

i’ve configured using gmail as for my server for sending out email “globally” like the following tutorial

But my server is hosting multiple domain. So, every domain email will send through above gmail, how can i config each virtual host using its gmail smtp server?

for example will use will use

i found a tutorial in ubuntu, but i’m using centos and webmin+virtualmin

how to do this in webmin?