If you want to convert PST to MBOX then I recommend you use CubexSoft PST to MBOX Converter to migrate Outlook PST files into multiple file formats. This is the most reliable software that allows Migrate PST files to  MBOX, PDF, EML, AOL, Gmail, Yahoo Mail, Office 365, and etc. You can add multiple files at a one-time conversion. A user with or without any technical background can easily complete PST file migration because of its simple GUI. Works on all versions of windows and mac OS.