You can try out the SysConverter for MBOX tool to complete the importing. With this tool, you can effortlessly import emails form MBOX to various IMAP Server accounts without any hassle. With its simple and user-friendly interface, it is easy to complete the importing without any hassle. You can try out the demo version of the software analyise its working process. For more info you can take reference of this blog – https://sysconverter.com/blog/import-mbox-to-fastmail/