If you want to Import MBOX to Outlook.com, then i suggest you eSoftTools MBOX to PST Converter Software. This software supports multiple file formats like - EML, NSF, WLM, HTML, MHTML, Gmail, EMLX, MSG, etc. You can convert any size of the file into multiple file formats without any kind of problem. This software works in all windows editions and older versions. It comes with a free demo version where you can convert 25 MBOX files without any charges.
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