Hi,
Prior to moving users to another Exchange Server you should always ensure you have set the required permissions for the BlackBerry Service Account (e.g. BESadmin). As an example the service account should be an "Exchange View Only Administrator" with Administer Mailbox Store, Receive As, Send As" permissions added at the server level. It is also worth checking that the permissions are delegated down to the mail store level. If all these settings are in place and it still fails it can be caused by the BlackBerry Account being corrupt. In this event you will have to create a new service account by following these instructions:
Switch BlackBerry Enterprise Server administration accounts
Thanks,
Gary