How to prevent mailbox access during Exchange migration?

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http://serverfault.com – I am doing a migration of mailboxes from one Exchange 2007 server to another server(new hardware, different server name, both on the same domain). What steps do I need to take to ensure that no users access the mailbox stores during the migration process? Disable port 25? Disable OWA? I will be using the move-mailbox exchange console functionality. (HowTos)