Importing MBOX files into Microsoft 365 requires choosing a suitable migration method because MBOX files cannot be imported directly through the standard Microsoft 365 mailbox interface. The right approach depends on the number of emails, archive size, and whether you need to preserve the original folder structure.
One option is to import MBOX files into a compatible email client, such as Mozilla Thunderbird, and connect it to your Microsoft 365 account through IMAP. You can then move the emails into the required mailbox folders. However, this process may take time when handling large archives.

Another approach is to convert MBOX files into PST format and use Microsoft's supported PST import process to transfer the emails into Exchange Online. This can be useful when migrating multiple archives or managing large amounts of email data.
Before starting, create a backup of your original MBOX files and test the migration with a small sample. After transferring the emails, verify message content, attachments, dates, and folder organization.

If you want to import using MacSonik MBOX to Office 365 more efficiently, you can also explore different migration utilities and compare their features, compatibility, and batch-processing capabilities. Choose a method that suits your mailbox size and technical requirements to help ensure a smooth migration.