I have just set up a Server 2012 machine with Exchange 2013. Everything is working good, except OWA. i can log into it fine from outside the network and receive emails fine. But no one can send emails. Whenever an email is sent, it simply never arrives at its destination. When using OWA inside the network, it sends just fine. But from outside, the message never arrive.
Could someone please point me in the right direction?