A method for mailbox migration includes creating a message ID map for the source and destination mailboxes, and storing the message ID map in a migration cache file that can also hold message ID persistence and message state indicator data. A precopy operation is performed while the source mailbox is still active, thereby allowing the time consuming process of message copying to be performed without interrupting user access to email. After email access is redirected to the destination mailbox, a final update is performed to bring the destination mailbox into a fully current state. Because of the precopy operation, the amount of required updating is minimized. By performing the updating based on message data stored in the migration cache file (and extracted from the messages themselves), the speed of the final update is further enhanced, since time-consuming message or header downloading is not required.