Go to Start » All Programs » SysTools Outlook to MBOX » SysTools Outlook to MBOX. A Check for Prerequisite window will appear.

Click on the "Open" tab to initiate the process of Outlook PST to MBOX Conversion.

Now, click on the "Browse" button to select the PST file to be converted.

After you click on the Browse button, given screen will appear. Select the path of the PST file to be converted.

Click on the second "Browse" button.

As you click on the second "Browse" button, the given screen will appear where you need to provide the destination path.

As you will select the Destination path the next screen will appear like the one shown below. Then, if you want to maintain the same Folder Hierarchy i.e., keep the same order of the folder which was there in your previous email client, click on the radio button "Yes" or click on "No" if you do not want the hierarchy to be maintained.

Now, click on the "Export" button to start the conversion process from PST file to MBOX format.

As the conversion process begins, you will see the Count Details of PST file.
It includes the details like current folder name, folder count, & email count during the conversion process.

Once the migration finishes, you will be prompted about its completion and the path of the converted file will be shown. Click "OK"
Now find the MBOX file in the same folder that you selected during the conversion process.

Navigate to the location where you saved converted Outlook PST data into MBOX format. Here you can find the output MBOX file.