There are many ways to convert PST to MBOX, but I suggest the third party tool KDETools, a PST to MBOX conversion software. Hope you find this information helpful.

Convert PST file format to MBOX for 20+ email clients including AppleMail, Opera, Thunderbird, etc.

Compatible with Outlook files in UNICODE and ANSI formats.

This software supports simultaneous batch conversion.

The program easily transfers PST file data from various email clients to PST files.

Support for all latest versions of Windows and MS Outlook.

READ MORE INFO:   https://www.kdetools.com/pst/mbox/