concretesubmarine.com/ FORUM

Manual solution to import MBOX file to Office 365
Here, I’ll configure my Office 365 account to Thunderbird and then import all my MBOX files. Below is the step-by-step process.

Phase1: Create Office 365 profile in Thunderbird
Firstly, open the Thunderbird in your system. (If not installed, you can install it from here).
Now go to the Thunderbird Menu option and click on the Account Settings from the dropdown menu.
Then, click the Account Actions button and click the Add Mail Account.
Now, enter your full name, Office 365 email address, and password in the following wizard. After that, click on the Continue option.
Next, select the configuration settings and click the Done option.

Phase2: Import MBOX files to Office 365 profile
First, we’ll create a new folder where we need to import MBOX files and Add the ImportExport plugin to Thunderbird.
Make a right on the left pane and click on the New Subfolder option.
Enter a name for the Subfolder and click Ok.
Now go to the Menu option and click on Add-on & Themes option.
Now search for ImportExportToolNG and add it to the Thunderbird.
Following, make a right-click on the left pane, click the ImportExportToolNG option and select Import MBOX files from the dropdown list.
Select the Import directly one or more mbox files option and click Ok.
Now browse the MBOX files to import and click Ok.

Greetings,
Peter