Sendmail to excahnge server

I am currently using sendmail and use smart relay to an exchange 5.5 server.  In sendmail.cf i have DS192.168.1.92 which points to an exchange 5.5 server.  We are upgrading to Exchange 2003 and the .92 server is going away. I tried changing to the 2003 exch server address DS192.168.1.24 and restarted everything but it now will not send. I added the IP address of the HP box to be allowed to relay from the SMTP Virtual server in Exchange System Manager.  The following is the error I get when trying to send out an email.  I kind of assume it has to do with DNS but why did it work with Exchange 5.5 and fail with Exchange 2003.

Message 19/21047 Mail Delivery Subsystem

Date: Wed, 28 Nov 2007 18:56:12 -0500 (EST)
To: root@tandc.
Subject: Returned mail: Service unavailable
Auto-Submitted: auto-generated (failure)

[Attach #1: ""]
[Type: text/plain, Encoding: 7bit, Size: 336]

The original message was received at Wed, 28 Nov 2007 18:56:12 -0500 (EST)
from root@localhost

   ----- The following addresses had permanent fatal errors -----
gbohrman@tncliving.com

   ----- Transcript of session follows -----
... while talking to 192.168.1.24:
>>> HELO tandc.
<<< 501 5.5.4 Invalid Address
554 Service unavailable

[Attach #2: ""]
[Type: message/delivery-status, Encoding: 7bit, Size: 293]

Reporting-MTA: dns; tandc.
Arrival-Date: Wed, 28 Nov 2007 18:56:12 -0500 (EST)

Final-Recipient: RFC822; jsmith@tncliving.com
Action: failed
Status: 5.5.0
Remote-MTA: DNS; 192.168.1.24
Diagnostic-Code: SMTP; 501 5.5.4 Invalid Address
Last-Attempt-Date: Wed, 28 Nov 2007 18:56:12 -0500 (EST)

[Attach #3: ""]
[Type: message/rfc822, Encoding: 7bit, Size: 433]

-- Start of included mail.
Return-Path: <root>
Subject: test 656
To: jsmith@tncliving.com
Date: Wed, 28 Nov 2007 18:56:11 -0500 (EST)

test 656

-- End of included mail.