Postifix Restritions

HI

I am hosting about 30 domains on my CentOS machine with VirtualMin. A problem I am facing is some users send emails (using scripts) and use FROM address other than what is of their domains .e.g. they sometimes use abc@gmail.com etc.

I want to restrict Postfix to allow FROM address only among the domains hosted on this machine with Virtualmin. I tried reject_unknown_sender_domain but it only rejects if the domain name doesn't exists anywhere in the world.

I would be much thankful if you can please help.