Blocking Outgoing Email to External Domains

We have 2 types of users in the organization, one who can send external emails other who cannot. How can I block users from sending emails to domains other then what they belong to? I have an option of using CPanel as SMTP relay and actual mailboxes are hosting somewhere else. So both option can work for me if I get an option to block certain users or I can block all users using "1" SMTP gateway to not able to send emails to external domains.