To convert OST emails to MBOX, you can use a tool like Advik OST to MBOX Converter. It safely converts all emails, attachments, contacts, and calendars from OST to MBOX without losing any data. You can select specific folders or convert the entire mailbox, and the folder structure and metadata are preserved. The resulting MBOX files can be opened in Thunderbird, Apple Mail, Eudora, SeaMonkey, and other email clients.
Steps:
Launch Advik OST to MBOX Converter.
Add your OST file(s).
Preview emails and select folders to convert.
Choose MBOX as the output format.
Click Convert to finish the export.