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To import MBOX file to Office 365 manually, you can follow these steps:

Export the MBOX file to a PST file. You can use an email client such as Outlook or Thunderbird to do this.
Upload the PST file to Office 365. You can use the Outlook desktop app or the Office 365 Import Service to do this.

To export the MBOX file to a PST file using Outlook:
Open Outlook and click on the File tab.
Click on Open & Export and then click on Import/Export.
Select Export to a file and then click on Next.
Select Outlook Data File (.pst) and then click on Next.
Select the MBOX file that you want to export and then click on Next.
Browse to the location where you want to save the PST file and then click on Finish.

To upload the PST file to Office 365 using the Outlook desktop app:
Open Outlook and click on the File tab.
Click on Open & Export and then click on Import/Export.
Select Import from another program or file and then click on Next.
Select Outlook Data File (.pst) and then click on Next.
Browse to the PST file that you want to upload and then click on Next.
Select the Import items into the following folder option and then select the folder in Office 365 where you want to import the emails.
Click on Finish.

To upload the PST file to Office 365 using the Office 365 Import Service:
Go to the Office 365 Import Service website.
Click on Sign in.
Sign in with your Office 365 account.
Click on Outlook Data File (.pst).
Click on Choose files.
Browse to the PST file that you want to upload and then click on Open.
Click on Next.
Select the Choose a folder in Outlook option and then select the folder in Office 365 where you want to import the emails.
Click on Next.
Click on Start import.
Once the import is complete, you will be able to access the emails in Office 365.

Greetings,
Peter