I'd advise using eSoftTools PST to MBOX Converter Software because you can search How do I export Outlook emails to MBOX and it is the best option. It worked very helpful to export Emails from Outlook to MBOX. Users can get instant online technical supports 24 hours a day, seven days a week. All clients have access to a free trial version that allows them to convert up to 25 emails per folder.
Read more:- Export Outlook emails to MBOX
First, decide if you want to convert the entire . pst file to mbox. Perhaps you only want to export selected emails, or an a specific Outlook folder.
Finally, select your messages or folders, choose MBOX as the target format from the MessageExport toolbar, and then click the "Export" button.
This May Help,
Peter