If wants to specially convert PST to MBOX file format then I would recommend to use EmailsGuru PST to MBOX Converter. It is specifically developed to convert PST file to MBOX, maintaining the original folder structure and preserves the email metadata during the conversion. The converted MBOX file can be easily accessible or import to all major email clients like Mozilla Thunderbird, Yahoo Mail, Opera Mail, Apple Mail and many others. It supports all versions of Microsoft Outlook and MBOX clients. It also offers a demo version to users, which convert 25 emails from a PST file to MBOX format for free.