Initially, install & launch GainTools Exchange Migration Software.
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Click on Add source button, select Exchange Server mailboxes and add login details.
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Preview Exchange mailbox items and select items using filters to migrate.
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Select Office 365 option and enter its login details. You can also select other options.
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Now, check map mailboxes option, if required. This will start the migration and offer you report of the process. Click the Finish button when all is done.
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