If you want to transfer PST file to MBOX, I suggest
KDETools PST to MBOX Converter. The software easily transfers PST files to MBOX and various file formats such as EML, EMLX, HTML, PDF etc. The advanced tool enables the user to convert PST files to MBOX formats such as Windows, Thunderbird Linux and Macintosh. The tool allows the user to convert selected items from PST to MBOX without data loss.
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