PST to MBOX freeware solutions don’t function in accordance with the users’ expectations, especially regarding the precision of output and keeping the integrity preserved. Try this professional converter instead.
There are few PST to Thunderbird Converter, but not all of them will convert the files with safety and precision.
Convert Outlook to Apple Mail Safer & Smarter Way
Adding Outlook to Mac Mail through file conversion technique using ‘PST Extractor Pro,” an extremely accurate and quick tool from USL Software.