I'm currently involved within a project where a new forest/domain is build. In this new forest first the workstations and user will be migrated followed by the back-end components. Therefore the users in the new domain need to have access to the Exchange mailboxes hosted in the current domain.

Therefore we created a new user account in the new domain. Secondly we disabled the user account in the current domain and assigned the permission Associated external account to the user account in the new domain.

With these settings the user can use his own mailbox as usual. However we notice that earlier assigned permissions to another user's mailbox are not functioning anymore when this right is assigned directly to the user. If the permissions are set on a Distribution Group the user can still access that resource.

Is there any possibility that the permissions assigned on a per user based can be preserved in this process or should we re-assing such permissions?
We are running MS Exchange 2003 in a domain 2003 (with domain level 2003) in forest running on forest level 2000.