There are many methods to convert PST to MBOX file format, but I will suggest safe and secure software PST to MBOX Converter. This tool enables all over 20 email clients supported by MBOX to use applications such as Eudora, Spicebird, Netscape, SeaMonkey and many more. The tool works fine with ANSI and UNICODE PST files. It is also supported by MS Outlook 2003, 2007, 2010, 2013 or 2016, 2019.

Click here ;
https://www.kdetools.com/pst/mbox/