I have several users who have left the organization and taken their systems with. The systems had been members of the domain, and had been connecting to an SBS 2003 server running Exchange. After leaving the organization they were moved to a pop connection offered by a third party. Everything worked well, they could email back into the original organization without any difficulty. As of mid May, they have been receiving the following error message. Your message did not reach some or all of the intended recipients.
Sent: 7/7/2009 12:24 PM
The following recipient(s) could not be reached:
'email@example.com' on 7/7/2009 12:24 PM
550 5.1.1 <firstname.lastname@example.org>... User unknown
Even if they try to email users that have been added to the old organization after they left, they get the same message. They can receive mail from the original organization, just not reply or create new mail into the it. All other email addresses appear to work fine. I suspect something within Outlook has tried to revert to the original exchange configuration, because the error message is almost instantaneous, but am at a loss as to where to look.