The best way to convert PST to MBOX is through using a special PST converter tool which has the ability of converting to MBOX. Begin by installing the program after which you add your PST file. All mailbox folders will be scanned and listed by a tool. All you need to do is just to pick the folders you desire to convert, select MBOX as your output format, and select out-put folder.
Click on Convert and the application will convert your Outlook emails (with attachment and folders) into a single MBOX file. This file can be subsequently imported on to email clients such as Thunderbird, Apple-mail or Eudora. To get satisfactory findings, BLR Tools is a preference of many users.

Click here: https://www.blrtools.com/pst-exporter-tool.html

MacSonik Outlook PST Converter is one of the most reliable tool to convert PST files into MBOX with or without attachments. This software can also convert PST files into different file formats like EML, MSG, PDF, HTML, MBOX, etc., and into various email clients like Office 365, AOL, etc. Users are allowed to save their converted files with their specific custom names. This PST converter tool for Mac has an option to remove duplicate emails from the conversion and migration process without losing the original folder hierarchy. It has multiple advanced features to customize the conversion process.

Know more- https://www.macsonik.com/pst/converter/

Users can now easily convert PST files to MBOX using the PST to MBOX Exporter tool which can easily convert single or multiple PST files to MBOX without any data loss. It is designed with the latest and smart technology that provides ease to users. It does not require any technical person to run. The GUI of the software is very user-friendly and designed with the latest and advanced technology. It offers a free demo version that helps in evaluating the functionality of the software. Visit here for more info: https://www.betavare.com/pst/mbox/