You should use the eSoftTools PST to MBOX Converter Software if you have a query about how to export emails from Outlook to mbox. This application is easy and quick to use. All of the properties of Outlook were preserved during the migration to Mbox. A free trial version of the software is available. You can export 25 emails from each folder in the free trial version.
For more information:- Export Emails from Outlook to MBOX