It's simple and easy to convert PST into MBOX. I recommend using eSoftTools Outlook PST MBOX File Converter (https://www.esofttools.com/pst-to-mbox-converter.html) software. This software can easily be used and all Outlook PST files can be converted to MBOX files. Split options are also provided to all users to divide the huge files into small multiple files based on the size. It can offer a free demo to all users to convert initial 25 items from each pst file to mbox. 
Read more:- Convert PST to MBOX For Free



 

In any case, if you are looking to convert multiple MSG files to PST format, then try ZOOK MSG to PST Converter and export your MSG files to PST format. It is the most reliable solution to convert MSG to PST with attachments in a single click. It has simple and easy to use interface of the utility which enables any novice user to perform MSG to PST conversion.
Overall it is a two-step conversion process to convert multiple MSG files to PST format in couple of seconds. Just follow the simple steps as described below: -

• Download ZOOK MSG to PST converter and install it in your system.
• Now, select MSG files or MSG folder.
• Choose desired file location.
• Click on Convert button.

Advantages of Using MSG to PST Converter

• It Converts MSG to PST with attachments in single click.
• Batch convert MSG to PST format at once.
• Preserves data integrity and RTF structure.
• Maintains folder hierarchy to keep all emails in same folders.
• Fully supportable to Outlook 2016, 2013, 2010, 2007, etc.
• Outlook Installation is not required.