Moving Mailboxes Across Forests

• $c = Get-Credential  <-- Enter your account for X domain
• $t = Get-Credential  <-- Enter your account for Y domain
• move-mailbox -TargetDatabase "servername\storage group\database" -Identity 'mailbox name' -GlobalCatalog servername -SourceForestGlobalCatalog servername -NTAccountOU "OU=XXXX,DC=domain,DC=com" -SourceForestCredential $c -TargetForestCredential $t

Make sure to transfer any X500 addresses over as well.