I set up an Exchange 2007 server in our Exchange 2000 Environment, and since have been unable to send outbound email from an Exchange 2000 server in a remote site. The remote site does not have a 2003 DC, only 2000. The following undeliverables are received on all outbound emails:
You do not have permission to send to this recipient.
<server.domain.com #5.7.1 smtp;550 5.7.1 Unable to relay>