I would like to suggest the easiest software to migrate PST to MBOX via
KDETools PST to MBOX Converter Software. This tool allows you to easily convert all email elements such as emails, notes, contacts, calendars, drafts, tasks, diaries, hyperlinks, email attachments, etc. PST to MBOX migration allows preview checking of mailbox data items & attachments before actual conversion takes place.
CLICK HERE: https://www.kdetools.com/pst/mbox/