Limit email delivery within its domain only !!!

Hello Every one !

I have query from one of my clients and am not sure how can this be done, Any help from the board will be highly appreciated.

The user is hosting a domain on my server and has many email address on it, now the issue is, he wants to restrict the mail flow only within his domain, that is the users should only be able to send mails from this domain to his domain only and no where else, BUT they should be able to receive mails from outside the domain !!!!!!!

If my query makes sense to anybody please assist.

Thank you all !