How to open Mbox files from Google Takeout 

Open Google Email Archive (.mbox) in Mozilla Thunderbird

Introduction

Archives of Google inboxes or Google Groups are typically exported as .mbox files which require a third-party application to open and navigate. We recommend utilizing Mozilla Thunderbird as it's free, secure, and relatively easy to use.

Follow the instructions below to open your .mbox file

Instructions

Instructions: 

Windows: https://drive.google.com/file/d/1084Fhpzk0OYMNc4Y9O2Jf6meJx1-okgU/view?usp=sharing

Mac: https://drive.google.com/file/d/1pdHVDiIdU2JbE5q3P-8qGjl8YUcNZ8vf/view?usp=sharing

4. Click the hamburger menu in the top right and select Addons and Themes

5. Search for ImportExportTools NG locate it from the list and click Add to Thunderbird

6. If prompted, click Add to confirm the installation

7. Return to your new account tab

8. Right-click on the name of your account, select ImportExportTools NG > Import mbox file

9. Select Import directly one or more mbox files and click OK

10. Locate the file on your computer and click Open

11. The messages may take a minute to load. You can now navigate the contents of the archive