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Search for ImportExportTools NG. Select Select Add to Thunderbird. Once installed, restart Thunderbird.
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To import mbox's MBOXs locally to into Thunderbird, select Local Folders in the left-hand pane to highlight it.
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Select Tools>ImportExportTools NG>Import MBOX files>Import directly one or more MBOX files>Select your exported MBOX file and click Open.
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You can select all messages in the imported folder and then drag and drop them to the corresponding folder in your destination account. Repeat these steps for all exported MBOX files.