Try MacMister OST Converter for Mac Tool which allows to convert OST file to MBOX and other formats such as EML, PDF, EMLx, HTML, MHT, ICS, vCard, CSV, MBOX, Yahoo, Gmail, Thunderbird, O365, IMAP, TXT, etc. The tool successfully convert multiple OST
files to MBOX format with attachments. Also, it supports all macOS and Win OS platforms.
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https://www.macmister.com/ost/
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