If you want to instantly transfer MBOX files with emails to Office 365 account, then you can try Advik MBOX Converter. It is such an amazing tool that offers a direct way to import MBOX to Office 365 account. You can transfer multiple MBOX files to Office 365 as per your needs and requirements. Also, this tool maintains a complete folder hierarchy of MBOX emails and transfers into Office 365 account securely.
For more info: https://www.adviksoft.com/blog/import-mbox-to-office-365/