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Do you want to know the process to open an OST file into MBOX? Then I would suggest you use the SysInspire OST to PST Converter tool. This is the most powerful software to convert your OST file to PST and 15+ other formats including  PST, Office 365, EML, EMLX, HTML, MSG, CSV, VCF, Yahoo Mail, IMAP Account, Gmail, Zimbra Desktop, and many more. This software is the best feature for the users to ease their work like email filtration and mass conversion. If you want to try this software, the free version is also available. 

Know more :-  open an OST file into MBOX

https://www.sysinspire.com/blog/how-to-convert-ost-to-mbox/

https://www.sysinspire.com/ost-to-pst-converter/