As a regular user, I would suggest going with Aryson PST Migration Tool. It migrates PST files to various file formats like PNG, JIF, JPG, DOC, CSV, etc. You can also save the PST files to Office 365, Yahoo, Thunderbird, Opera, Live Exchange, and other email clients. The utility provides an option to save email attachments separately and remove duplicate items. It converts the PST file with email, contacts, calendars, and notes with no data loss.

Here’s how you can convert the PST files to MBOX:
Step 1: Download and run the Aryson PST Migration Tool.
Step 2: Select the Browse button and add the PST file.
Step 3: Mark the tree-structured folder and preview the emails.
Step 4: Click the Next button and choose the saving format.
Step 5: Choose the location to save the file and tap on the Convert button.

Know more: https://www.arysontechnologies.com/pst-migration/