new organizational unit users not able to receive email outside my domain

I have a couple of users that I recently created in a new OU and now cannot receive email outside of the domain.  I have one exchange server and host multiple companies email and this is the only one I'm having trouble with.  I setup a new organizational unit in my active directory and setup two users in that same unit.  I then created a exchange store in my exchange console and mounted the store.  Ran some test mails and made sure they could login to the network etc, etc.  But now they are telling me that their customers are getting kick back emails saying access is denied (error 550) and that the domain doesn't exist.  I have checked my MX records and everything seems to be fine there.