restrict users to internal email only

Hello there,

I am somewhat stuck - maybe you can point me in the right direction.
I want to restrict some users, so they can only send mails to other
users on the kolab system. Didnt find any documentation of how to do
that as "Directory Manager" so I tried to restrict the users on
postfix-level[0]. Although I followed the manual, so far, I can still
send emails to whatever domain I like with the testuser.
Sending restrictions are the priority, but it is also ok to completely
lock down the user to internal email only.

thank you very much
Thomas



[0] http://www.postfix.org/RESTRICTION_CLASS_README.html