#5.7.1; smtp 550 5.7.1 Replay Access Denied

 Hi,

I have a user using outlook 2010. He created a group with his vendor emails. Everytime he sends out email to the group, one particular recipient get kicked back all the time with the error

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

      Subject:  xxxx

      Sent:     6/5/2013 11:05 AM

The following recipient(s) cannot be reached:

      XXXX on 6/5/2013 11:05 AM

            You do not have permission to send to this recipient.  For assistance, contact your system administrator.

            <our domain> #5.7.1 smtp;550 5.7.1 <recipient email>: Relay access denied.>

However, he can send email to this recipient individually just fine. Will anyone tell me why is this happening? Thanks in advance.

FYI, we have exchange server 2003, and client OS is windows 7 pro 64b

Bruce Wayne