I've heard back from another MVP, the question comes up, do you know how these users were added to the original SBS 2003
Has Exchange on the SBS 2003 server been removed yet? If not that would be my priority.
If that does not resolve the issue, then in SBS 2008 Open the Exchange Manager Console >Expand Recipient Configuration >Click on Mail Contact
I would make record of all contacts there, then delete them. Then go to the affected users Mailbox Exchange Manager Console >Expand Recipient Configuration > Mailbox > Right Click on one of the affected users > Properties > Mail Flow Tab > > Click on Delivery Options > Properties button. Remove any forwading.check boxes
I would then restart the server and when it comes back up...see if you still have the issue.