Error 550 5.7.1. Relaying denied. Proper authentication require

Hello Experts,
I have a user that is getting the following message:

Your message did not reach some or all of the intended recipients.

      Subject:      
      Sent:      9/27/2007 12:34 PM

The following recipient(s) could not be reached:

      amy@wholesomechoicepetmarket.com on 9/27/2007 12:34 PM
            You do not have permission to send to this recipient.  For assistance, contact your system administrator.
            <usrmcpdc0021.newcodistributors.com #5.7.1 smtp;550 5.7.1 <amy@wholesomechoicepetmarket.com>... Relaying denied. Proper authentication required.>

We are using Exchange 6.5 SP2 on a Windows 2003 SP1 Server environment. Can you please help!
Thank youStart Free Trial