You can utilize the Free OST to MBOX Converter software from Perfect data solutions that instantly removes all errors from your Corrupt OST file and make it again accessible. It additionally provides the feature to convert the Exchange OST file to multiple file formats like- MBOX file format. A free demo is offered by PDS OST to MBOX Converter which has the capabilities to convert 30 items without paying any charges.

Features of PDS OST to MBOX Converter Software:-
Export multiple emails along with attachments from OST Files to MBOX file.
Scans and preview emails, contacts, calendars, task, notes etc.
No need of Exchange Server environment for OST Conversion to MBOX.
No size limitation: Any size OST file can be converted into MBOX file.
Convert selective mailboxes or folders of OST files to MBOX files.
Messages filtering available; filter the emails by date selection and convert into MBOX.
Auto searching .ost location from your machine hard drive to migrate emails.
Capable of extracting password protected and highly encrypted OST files.
Supports windows versions upto 11 (32 and 64 bit) and MS Outlook versions upto 2021.

Read-More: https://www.perfectdatasolutions.com/en/ost/ost-to-mbox-converter.html