To import MBOX to Exchange Online (part of Microsoft 365), you just need to create a new user account in the Microsoft 365 admin center. When you create the user, Exchange automatically creates a mailbox for that account.
Here are the easy steps:
Sign in to your Microsoft 365 admin center at https://admin.microsoft.com.
Go to Users > Active users.
Click on “Add a user”.
Fill in the user details like name, username, and password.
Assign a license that includes Exchange Online (like Microsoft 365 Business Standard or E3).
Click Finish adding.
Done! The user now has a mailbox in Exchange Online, and they can access it through Outlook Web (OWA) or the Outlook desktop app.