Moving mailboxes, Exchange 2010 way

@Christopher : The –AllowMerge behavior on the move-mailbox was a simple patch for the lack on online availability during the move. Since now the moves are online by default, this behavior should not be needed anymore. Any cross-forest deployment can be set for rich co-existence between the 2 forests, so that mailboxes on either forest have full access to all the features on the other forest (like free/busy, calendaring, …), so even if half of the mailboxes have been moved, there no reason to keep the migrated mailboxes on the source forest (it only complicates the user’s life, since when the account is moved by exchange AutoDiscover will prompt Outlook to reconfigure itself, while keeping the mailbox on the source will force them to manually fix the client when the source forest is being decommissioned). So, no, there’s not a way to prevent the source mailbox from being deleted. There will be something in SP1, but the left behind mailbox will be for data restore purposes only – in case the target mailbox suffers something terrible right after the move.