550 5.1.1 RESOLVER.ADR.RecipNotFound; not found - Exchange 2010 - Postfix

Hello,

I have a LAN that has an Exchange 2010 server and a Linux Postfix server

Trying to send a message from an account that is on the Exchange server to an account
 that is on the Linux server I get the following error:

Diagnostic information for administrators:

Generating server: Server1.Domain.local

user@def.com
#550 5.1.1 RESOLVER.ADR.RecipNotFound; not found ##

Original message headers:

Received: from Server0.Domain.local ([192.168.217.10]) by
 Server1.Domain.local ([192.168.217.11]) with mapi id 14.02.0342.003;
 Fri, 17 May 2013 23:43:23 -0400
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding:binary
From: "User" <user@abc.com>
To: "user@def.com" <user@def.com>
Subject: TEST @ 2343
Thread-Topic: TEST @ 2343
Thread-Index: Ac5TedhzMEpuDnfBSIeFpuE8fSaBEQ==
Date: Fri, 17 May 2013 23:43:22 -0400
Message-ID: <E3336DF928658B45AD4B60C756612AD53C3538@GEMWIN0000>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator: <E3336DF928658B45AD4B60C756612AD53C3538@GEMWIN0000>
MIME-Version: 1.0
X-Originating-IP: [x.x.x.x]

I am able to successfully send messages to other accounts that are on the Exchange server.
I am able to send mail to the Linux accounts from outside the LAN no problem.

Any ideas?

Thank you for your time,

Don