I suggest you try a manual method to export PST to MBOX file format. Therefore choose PST to MBOX Converter. This software supports MBOX files from more than 20 email applications, including Apple Mail and Pocomail. Eudora, Spicebird, Netscape, SeaMonkey and many more. This software can export all recovered data in a multiple file format such as MSG, EML, HTML and EMLX. It also offers the solution to restore and export MBOX to Office 365, Live Exchange Server.
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Start Migrating Outlook PST files into MBOX format with the help of PST to MBOX Converter that easily exports Outlook PST files to MBOX without any data loss. It is very easy to use this tool for technical as well as non-technical users. The software allows migrating Outlook PST file generated from any Outlook PST supportive platform. This tool moves the Outlook PST emails include all attachments. Through this converter, you can also merge a large amount of Outlook PST files into individual MBOX format. Refer:- http://www.softwaretoconvert.com/pst-to-mbox/
Convert Outlook email (PST or Exchange) to MBOX format
Outlook 2010 or newer: Click on the small triangle below the MessageSave Ribbon button and select. "Apple Mac Export -> Export Messages". ...
Click "Start Export".
That's it! MessageSave will export your email messages to a folder on your desktop.
I would like to suggest PST to MBOX converter software that is helpful to convert PST file to MBOX file format. this converter helps to import and save data in various file formats like EML, EMLX, HTML, MSG. User can use batch conversion, which converts single and multiple PST files to MBOX format. Supports all versions of Windows like 10, 8, 7, XP. Capable in ANSI and UNICODE formats.