Not receiving e-mails from outside after setting up Exchange 2013

Hi Guys,

I have just setup an brand new Exchange 2013 which working almost 80 % if i may put it that way.

My users are sending e-mails without and problem.The issue i have is users cannot receive e-mails from outside the organisation When we send emails we get bounce back that saysrecipient address rejected: User unknown in
virtual mailbox table'

My Environment

dc on server 2012 r2exchange (both cas and mailboxon one server) on server 2012  r2.

Thanks

Kassoka