Outlook 2003 removed from Exchange

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.

Subject:      test
Sent:      7/7/2009 12:24 PM
The following recipient(s) could not be reached:
      'user@oldorganization.com' on 7/7/2009 12:24 PM
            550 5.1.1 <user@oldorganization.com>... 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.
aweltnerAsked:
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MesthaCommented:
The problem you have is that because you are not using Exchange for those users, you have no idea what is happening to the message. It doesn't look like an Exchange message, so would tend to point to a DNS lookup problem - possibly the server the messages are being sent to are sending the message to the wrong host.
Troubleshooting it is almost impossible because you don't control the server the message is being delivered from.

The most I can suggest is to confirm there is no connection to your server using SMTP logs. However if you collect email via POP3 then that isn't going to be possible. A call will have to be made to the ISP who those users send email through to find out what is happening.

Simon.
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