Recently I have involved in a cross forest Exchange 2007 to Exchange 2010 cross forest migration, and I had an issue on the converted mail user of source forest after the successful cross forest move. In my case the migration was from resource forest to account forest (Linked Mialbox to User Mailbox Migration)
This issue happens when initiate a remote legacy mailbox from Exchange 2007 forest to an Exchange 2010 forest. The mailbox move finishes as normal but we will find some corrupted data on the RecipientTypeDetails attribute on the source forest Mail-User and the value is updated with “33554432”.
And when you open the properties, the below warning shows up,
If you click on OK button, it will correct it but the email address tab may be adjusted according to the policies and the primary SMTP address may be updated with the external email address value etc.
This issue is affected only when you have updated your Exchange Server 2010 with the Service Pack 2. To fix it, please update with Update Rollup 1 for Exchange Server 2010 SP2
The update rollup1 fixes many mailbox move issue, and it is recommended to install it if you are in middle of migrations. And see the take care notes when you upgrade with Rollup1 here