How to restrict users sending/receiving mails to other domains in NMS

In an Extranet where five domains are there, how can I prevent users to send mail only between three domains from rest of the two domains, they should not have access. sometimes sending mail fails from the same host with multiple domains.
[252 byte] By [709058] at [2007-11-25 7:09:57]
# 1
There is a provision in admin module where by the admin can deny or grant access to a user or a group of users for accessing or being denied.It could be a service ,username or domain name to customize the users previlieges.
709454 at 2007-6-29 17:41:54 > top of Java-index,E-Mail, Calendar, & Collaboration,Sun Java System Messaging Server...