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Read More:- Convert MBOX to Office 365

Step #1 Convert MBOX to PST
Open the Thunderbird email client and select the required folder from the left panel of the screen.
Go to right panel and select all the emails in that folder with ‘Ctrl+A.’
There is a blue highlighted message. Right-click on the highlighted message and select the Save As option from the displayed menu
Browse the location to save the folder and click ‘select folder.’
Once the folder is saved, minimize the email client – Thunderbird and open Outlook
Right-click on the folder in which the saved data is to be moved (emails from Thunderbird) and select the option – ‘Create a new folder.’
Next, name the folder
Go to Thunderbird again. Drag the saved folder and drop it in the recently created folder in Outlook
Save the folder in PST format
Step #2 Export PST file to Office 365 account
Export PST to Office 365 with the help of Network Upload:
Install Microsoft Azure AzCopy on the PC and create a copy of SAS URL
Upload recently created PST file
View all emails in PST file
Make a new mapping of PST data file and generate a PST import job in Exchange online account.
office.com/setup365 lets you manage your office account with all the features that you will require to manage your subscriptions, renewal, billings and make new purchases.
Go to office.com/myaccount and select Sign In.
Enter the email address and password. This might be your personal Microsoft office myaccount, or the username and password you use with your work or home and business account.
Choose the App Launcher and then choose any Office app to start using it.