邮件无法relay？

sh41667

From: System Administrator
Sent: Thursday, January 31, 2008 9:35 AM
To: user
Subject: Undeliverable: subject
Your message did not reach some or all of the intended recipients.
      Subject: subject
      Sent: 1/31/2008 9:35 AM
The following recipient(s) could not be reached:
      user@abc.com on 1/31/2008 9:35 AM
            You do not have permission to send to this recipient.  For assistance, contact your system administrator.
            <server  #5.7.1 smtp;550 5.7.1 Unable to relay for user@abc.com>

很奇怪，abc.com和abcxyz.com都用一个connector,发到abcxyz.com的都正常，发到abc.com的都出现上述问题

会是什么引起的？

谢谢