Hey, I experienced the similar problem a few months ago. I tried a few options and I went for Softaken MBOX to PST Converter to Convert MBOX files to PST and it worked really well for me. The conversion was clean, no data lost and it managed my enormous MBOX files without crashing. Emails, attachments and folder structure all came through intact. The UI is relatively clear forward, so you don't have to be a tech geek. And it also offers batch converting, which saved me a lot of time. If you want a hassle-free solution, definitely try it.