Exchange 2003 SP2 - UCE Host Blacklisted Error

We are running Exchange 2003 SP2.

When our users send email to the domain ***.is they get the message below as a bounce back. No filtering is done on the Exchange SMTP server so I'm not sure where exchange is blocking this email. Any ideas?

Subject:        Undeliverable: Checking in on Email address
Your message did not reach some or all of the intended recipients.
      Subject:  Checking in on Email address
      Sent:     2/22/2007 10:19 AM
The following recipient(s) could not be reached:
      *****@XXXX.is on 2/22/2007 10:20 AM
            You do not have permission to send to this recipient.  For assistance, contact your system administrator.
            <server.domain.net #5.7.1 smtp;500 5.7.1 <server.domain.com[xx.61.203.xxx]>: Client host rejected: Hostname UCE blacklistedx>
      *****@XXXX.is on 2/22/2007 10:09 AM
            You do not have permission to send to this recipient.  For assistance, contact your system administrator.
                <server.domain.net #5.7.1 smtp;500 5.7.1 <server.domain.com[xx.61.203.xxx]>: Client host rejected: Hostname UCE blacklistedx>
 
Start Free Trial