You can use the Office 365 Migration Tool to backup your entire mailbox data. This software is designed to help businesses and individuals migrate their email, contacts, calendars, and other data from on-premise systems or other email services to Office. This tool simplifies the migration process by automating tasks such as mailbox mapping, user account creation, and data transfer, reducing downtime and minimizing the risk of data loss. It also offers features such as pre-migration checks, progress tracking, and reporting to ensure a seamless and successful migration experience.