If you are serching for effective ways to open the MBOX files into MS Outlook for windows then you should give it a try to Cigati MBOX Converter. It has an amazing User interface that allows to easily convert the MBOX files into PST file format. I recently opt this tool to import the MBOX files into MS Outlook. It made it so easy and reliable that I haven't faced any difficulties to import all the MBOX files into Outlook in a single go.
Additionally, it facilitates to convert it into multiple formats like PDF, JSON, PST, EML, EMLX, DOC, DOCX, etc. If you want to test the software performance then you can download its free demo version that will allow the conversion of up to 50 emails.