If you are looking for solution to migrate Outlook PST files into MBOX format then you can try most reliable and easy to use ZOOK PST to MBOX Converter  software. It is developed with highly advanced algorithm which makes it a powerful and easy tool. This tool enable users to convert PST files to MBOX format with an attachments and allows to save them in different MBOX supported email clients like Mozilla, Thunderbird, Opera Mail, Spicebird, Apple Mail, etc. Users can export big sized PST files into MBOX format without any restriction and customize the size of resultant MBOX file.

For more info- https://www.zooksoftware.com/pst-to-mbox/