For converting PST to MBOX, I have a best solution for you, So you can Download PST to MBOX Converter software. PST to MBOX converter is a simple and fast solution that converts and supports over 20 email clients like Entourage, Opera, Netscape, Thunderbird, Apple Mail etc. Convert password-protected and highly encrypted files. This software is compatible with Outlook versions 2016, 2013, 2010 and 2007 etc.
Use the best PST converter tool and converts all the PST files into MBOX format. A user can convert PST files to several other file formats including PST, MSG, PDF, MBOX, etc. It has a feature of bulk conversion of PST files hence, multiple PST files can be converted to MBOX format at one time. The tool transfers PST files along with the attachments without any change in data structure and folder hierarchy. Refer:- http://www.download.adwages.com/outlook-pst-conversion/