I have an old OST file from my previous Outlook account, and I need to convert it to MBOX format so that I can access my emails in Thunderbird. Unfortunately, Outlook is not connected to the Exchange server anymore, so I can not open the OST file directly. Can anyone suggest a safe and reliable method or tool to convert OST to MBOX without data loss (emails, attachments, folders, etc.)? Also, is there any free or manual way to do this, or do I need to use a professional OST to MBOX converter?