You should use the eSoftTools PST to MBOX Converter Software to export emails from Outlook to MBOX. It has the capability of exporting emails from Outlook to Mbox. This application is easy to use and understand. All of the data's attributes were preserved during the pst to Mbox conversion. A free trial version of the software is available. You can export 25 emails from each folder in the free trial version.
Know more:- Export Emails from Outlook to MBOX
