Email failing to make it to a mailbox with in a domain.

Here is my issue. I can email from outside of the domain(1) to this one
mailbox in exchange. Now when I email from this one certian domain(2)
it never makes it to the mailbox. Now from that certian domain(2) I'm
able to send an email to another mailbox with in the domain(1). Now I
know email can make it from Domain(1) to Domain(2) and from Domain(2)
to Domain(1). But it won't arrive to the right mailbox. It never makes
it. I have checked everything in exchange even tracked it through
exchange, plus checked the spam filters to make sure the Domain(2)
wasn't being block locally on the users machine. I've made it to a
brick wall. Does anyone ever heard of this happening or seen it. Need
some help on this one.