[SPAM] Prevent certain users from send emails outside the local domain.

Hi,

I am required to prevent certain users on my Qmail Server from sending 
emails to outside domain.
In my understanding, only IP Addresses can be prevented from relaying! 
But that's something which does not work in my environment considering 
we have DHCP setup.

Any help will be appreciated.

Regards,

!DSPAM:4fd1ce5434201875735209!