I will suggest you most useful PST to MBOX Converter for hassle-free and instant solution to convert 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 or migrate batches of PST files into MBOX file format for email clients like Mozilla, Thunderbird, Apple Mail, Opera Mail, Netscape, Netscape, etc. The software comes with a demo version tool, you may try this well developed tool.
For more info- https://www.zooksoftware.com/pst-to-mbox/