Exchange mail routing between two forests

I am migrating users/mail between forest A ( and forest B (  Forest A uses Exchange 2003, and Forest B uses Exchange 2007. Once pilot users are migrated to the new forest, they need to receive email for both email addresses ( and  This will be done by adding the appropriate proxy addresses.  The remaing users that have not been migrated still need to receieve email from only the address.  Inbound internet mail will go to an SMTP gateway that directs mail to the appropriate mail server.  Once the pilot group is migrated, how will they receive email for the address since that mail will be sent to the maill server in domainA.    Would configuring forwarding on each migrated mailbox be the simplest way to do this?
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florin_sConnect With a Mentor Commented:

You can use the Move-Mailbox command. Here is a good tutorial on how to do cross-org mailbox moves:

Here is an article on how to setup a shared SMTP name space in Exchange 2007:

After you set this up, you will need to change your MX records to point to the Exchange 2007 server.

To ensure mail flows properly after you migrate a user, you should create hidden contacts for all your mailboxes. Those contacts should have the SMTP address After you migrate a mailbox, set it to forward to the contact. This will ensure that users still on the Exchange 2003 server can send mail to users migrated to the Exchange 2007 server.
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