I know this is possible because this is how we are set up. I dont know how it was set up specifically but I can tell you it is possible.
I know that all i have to do is change the domain of thier address.
Example:
user1@yourdomain.com = internet email access.
user1@host.name = no internet email access.
The users that only have @host.name can only send and recieve emails in house. I am not sure if thats the way everyones is set up, but I thought id try to help.