Cross-forest migration to where object and mailbox are in different domain trees.

I need to do a cross-forest migration of the AD object to one domain tree and the mailbox to the other domain tree, Not even sure if the product can do so out of hand. Most probably a manual intervention is needed as it will then be a linked mailbox.  To define it better the Ad object and mailbox need to be migrated to different domain trees in the Target. The Source object and mailbox is in the same domain\forest. Any advise will be appreciated.