I will suggest you most useful PST to MBOX Converter for hassle-free and instant solution to convert to PST files to MBOX format in a single process. I have personally tried this utility. It is fully secure and easy to use for all kind of users either you have technical knowledge or not! It does not matter how much big sized files you have! The software easily convert batches of PST files into MBOX file format so that users can easily access mailbox data of Outlook into MBOX supported email clients like Thunderbird, Mozilla, Mac Mail, Netscape, etc. This tool also comes with a demo version, You can directly download from the official website. https://www.zooksoftware.com/pst-to-mbox/