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There are a bunch of ways to do this. As far as existing message go, you may want to make a PST file and save it as a backup. Then you can import the PST into the main Exchange account, so that the user has a history of all sent messages. Then set the secondary email account to forward messages to the primary account, so that the user just has to watch a single inbox. The user will be able to send messages from either account, using a pulldown menu to select which address the email is coming from.