MBOX file format is supported by Mozilla Thunderbird and many more other email clients, we all discriminate fine individually email client has their individual file procedure to store user data. MBOX also has its own file structure to store user info in the Outlook PST file. Furthermore, the MBOX file is solid to migrate and have more hazards of the shift to Outlook PST file. You can try a professional App like GainTools MBOX to PST File Migrator. I also used this software and would suggest you same change your MBOX file.