Try the esoftTools PST to MBOX Converter Software it performs very simple and smoothly during the conversion PST to MBOX it will show a live preview email, contact, and attachments. This software support different emails clients like- Apple Mail, Eudora, Netscape, Opera Mail, Sea Monkey, and various other email clients. The demo version is available for all users in the demo version you can convert 25 emails per folder.
Read more:- PST to MBOX Windows
If you want to convert your Outlook PST file to MBOX file format, you can try this PST repair tool. It can easily save PST files in multiple file formats like MBOX, DBX, EML, PDF, MSG, RTF and HTML. With the help of this tool, the user can also move Outlook PST data directly to Office 365 mailboxes. The tool supports all versions of MS Outlook, Office 365 and Windows OS. Further information on the tool can be found at: https://www.datarepairtools.com/outlook-pst-repair.html