Cross-Forest Migration and Exchange 2010 "hosting mode"

I'm consulting for a large company with 2 forests (Forest A and Forest B) running 7 domains - all Exchange 2003.

I am creating a new forest, Forest C.

All users, groups, computer objects and contacts are being migrated from all domains in Forest A and Forest B into a new domain in Forest C.

My customer wanted to know if it was possible to move the users mailboxes "into the cloud" during the migration a forth forest, Forest D by an Exchange hosting provider running Exchange 2010 "hosting mode".

Here is a copy of the response I received from Microsoft on the matter:

Hi Clint,

Thanks for your update.

So, here I would like to re-understand your concern: there are four forests A, B, C and D(D forest is installed with hosting mode Exchange 2010 ). You want to confirm that whether it is possible to migrate user accounts, groups, computer accounts and other Active Directory objects from forest A, B to forest C while migrating mailboxes to forest D. Please let me know if there is any misunderstanding.

If my understand is correct, I would like to let you know that it is not recommended migration because of the following reason:

1. If we migrate the mailbox and Active Directory account to different forest, we have to create linked mailboxes. However, Exchange 2010 which is installed in hosting mode don't support "resource forest". For example, based on the result of many tests, if we create a mailbox in forest D for the user in forest C, this user is not able to see others in Global Address List while others are able to see him/her in Global Address List.

2. Hosting mode Exchange server is designed for multi-tenant. It is designed for those companies and organizations which haven't Active Directory and Exchange. So if we migrate mailboxes to the forest which holds hosting mode Exchange server, I believe there will be a lot of unexpected issues and inconvenience.

For your reference, I would like to share the following article with you:

Multi-Tenant Support
http://technet.microsoft.com/en-us/library/ff923272.aspx

Please feel free to let me know if you have any concerns. I'm looking forward to hearing from you.

Best regards,
Leo Qin
Partner Online Technical Community

Hope this sheds some light on the matter for anyone facing the same scenario.