I have an old OST file from my previous Outlook account, and now I need to access those emails in Thunderbird, which supports MBOX format. Unfortunately, Outlook is not connected to the Exchange server anymore, so I can not directly open the OST file. Can anyone please suggest an effective and safe way to convert OST to MBOX? I am looking for either a manual method (if possible) or a reliable tool that can do the conversion without data loss, including emails, attachments, and folder structure.