FW: email from one specific account to another go into a blackhole

I have two users, we will call them User1 and User2 for the our discussion…
When user1 forwards a message to user2 it never arrives to user2, neither their Blackberry or their Outlook account. I checked for the message on the exchange server and it shows that it was delivered to user2, but alas it never arrives. I have checked the rules and everything else but cannot figure out why messages are not getting through. Now here is the kicker – New Messages and replies from user1 to user2 show up just fine. AND If user1 forwards a message to any other user such as user3 it it delivered without issue.

WHAT could possibly be causing this scenario?