NDR's with Exchange 2007

I have a Windows Exchange 2007 Std Ed server.  We are currently using a POP connector to download email from our Email provider, but we are sending email directly from the Exchange server.  Intermittantly, some of my users are receiving NDR's.  I have included an example with the identifying details changed.  jdoe@notarealdomain is the receiver, jdoe@123.com is the sender, mail.123.internal is my internal exchange server (internal domain different from external domain.

After doing some research it looks like the cause may be due to my external mail IP being different than the IP associated with my MX record.  Does that sound correct?  If so, is there a way around this problem?



Delivery has failed to these recipients or distribution lists:

John Doe
Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.

The following organization rejected your message: mail.tomahawk.ca.

  _____  

Diagnostic information for administrators:

Generating server: 123.internal

jdoe@notarealdomain.com
mail.tomahawk.ca #501 5.7.1 <jdoe@123.com>... Sender IP must resolve ##

Original message headers:

Received: from mail.123.internal([172.16.1.8]) by
 mail.123.internal([172.16.1.8]) with mapi; Fri, 14 Mar 2008
 12:57:25 -0400
From: Jim Doe <jdoe@123.com>
To: John Doe<jdoe@notarealdomain.com>
Date: Fri, 14 Mar 2008 12:57:25 -0400
Subject: coats
Thread-Topic: coats
Thread-Index: AciF9HpLYx2wO0kTSoaFXgC5VK53TQ==
Message-ID: <E59CA236AC55D240B558133C94E5BDD0AF0ED887@mail.123.internal>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
acceptlanguage: en-US
Content-Type: multipart/alternative;
      boundary="_000_E59CA236AC55D240B558133C94E5BDD0AF0ED887hcexchangehalib_"
MIME-Version: 1.0