After migration old e-mails fail on reply.

I migrated an organization from Windows2000 SBS w/exchange2000, to Windows2003 Standard Server w/Exchange2003.

There is no real migration path so I ended up creating all the user accounts and mailboxes in the new organization then used Exmerge to export the old mailboxes from the old system, and then used Exmerge to import them back to the new mailboxes.
I changed the MX record to the new server IP address and everything looks good. OUWA works fine, Outlook clients internally connect and have no problem. All is well.

Except, when a user tries to reply to one of the old mails that were migrated we get the following.

Your message did not reach some or all of the intended recipients.
Subject:      RE: AC Adapter for Compaq Laptop
Sent:      10/26/2004 8:31 AM

The following recipient(s) could not be reached:

  username on 10/26/2004 8:32 AM
  The message could not be delivered because the recipient's destination email system is unknown or invalid. Please check the address and try again, or contact your system administrator to verify connectivity to the email system of the recipient.
  < #5.1.2>

Have any ideas as to where to look for this issue? The old domain name and new domain name are the same. Only the resolved IP address changed.
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moduloConnect With a Mentor Commented:
PAQed with points refunded (125)

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jlangerudAuthor Commented:
After further investigation we found this:
 We had run the Microsoft Office 2003 “Save my settings wizard”, before joining the new domain on the workstations. Then ran the import after joining the domain. This of course moved the outlook.ost to the new user. The chched mode had also moved the Reciently resolved To: addresses. (Type in the first few lerrers of the name and it populates the address.) These e-mail addresses were still tied to the old server name and caused the failure.

We deleted the old .ost files and created new ones. Since outlook had no cached names it had to go to the server and resolve it again and all was again, well with the world.

Thank you for your time.

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