Are you finding a solution to convert OST files to MBOX files? So there are simplest ways you can use to do conversion. I would suggest the SysInspire Conversion OST to PST tool. Which is a very powerful conversion tool. By using this tool you can convert OST to PST file easily and the 15 additional formats including  PST, Office 365, EML, EMLX, HTML, MSG, CSV, VCF, Yahoo Mail, IMAP Account, Gmail, Zimbra Desktop and many more. This software is easy to use and anybody can use this software without the knowledge of technical background.

Know more :- Convert OST files to MBOX files