In Exchange 2010, before moving a mailbox cross-forest, it is necessary to ensure the corresponing AD account has been moved to the target forest. Otherwise, the mailbox move process failes. So, the following steps focus on how to move AD accounts from the legacy Exchange forest to the Exchange 2010 forest and then how to move mailboxes afterwards.
Step 1: Move AD account
Note: Please perform the following steps in the target forest where Exchange 2010 server is installed.
1. Open Exchange Management Shell, and navigate to the folder where the Prepare-MoveRequest.ps1 script stores.
2. Type the following command and press Enter:
$cred = Get-Credential
Note: You will receive a pop-up window for entering the credential. Please enter the credential of the Administrator account in the source forest.
.\Prepare-MoveRequest.ps1 -Identity <e-mail address> -RemoteForestDomainController <FQDN of DC in source forest> -TargetMailUserOU <OU name> -RemoteForestCredential $cred
Note: Please replace <e-mail address> with the e-mail address of an AD account in the source forest.