An MBOX file is an archive of email messages based in UNIX systems, similar to the PST archive, that is used from Microsoft Outlook to store emails, calendars, tasks, etc. In fact, an MBOX file contains email messages that are stored in a concentrated format where each message is stored after another, starting with the "From" header.
The MBOX file type, is used today by Google to store your Gmail messages into a single .mbox archive, but you cannot open the MBOX file in Windows without using a third party program. After doing some research, I decided that the best free program to open MBOX files in Windows is the Thunderbird email application.
* Note: If you own a MAC computer, then you can open MBOX files, by using the Apple Mail application.
This tutorial contains detailed instructions on how to open a .MBOX Mailbox file (e.g. Google MBOX) in Thunderbird (Windows 7, 8 or 10).
How to Open Mailbox (.MBOX) files in Thunderbird.
1. Download and install Thunderbird Email Application on your system (if you haven't already installed it).
2. Launch Thunderbird. *
* Note: If you 're already using Thunderbird as your default email application, skip to step-10
3. At 'Add Account' screen click Cancel and at the next screen, click Skip Integration.
4. If you 're prompted to create an email address in 'gandi net', unselect the "gandi.net" checkbox and click I think I'll configure my account later
5. If you 're launching Thunderbird for first time,* click Newsgroups under "Create an account" options.
* Note: If you don't see the screen below: Click on the Thunderbird's main menu 
and select Options > Account Settings. Then click on Account Actions and choose Add Other Account > Newsgroup Account.
6. Type a symbolic name (e.g. "user") and a symbolic email address (e.g. user@example.com) and click Next.
7. At "Newsgroup Server" box type a symbolic name (e.g. "Server") and click Next.
8. At "Account name" box type a symbolic name (e.g. "User") and click Next.
8. At the last screen click Finish.
10. From "Account Settings" (Options > Account Settings) select and copy the "Local Directory" path for the "Local Folders"*
* Note: The "Local Folders" is the folder where Thunderbird stores its information and can be found at the following location on the disk:
- C:\Users\%username%\AppData\Roaming\Thunderbird\Profiles\<xxxxxxxx.default>\Mail\Local Folders\
11. Quit Thunderbird.
12. Open Windows explorer and paste the path address at Explorer's Address bar and press Enter.
13. Then copy and paste (or drag and drop) the mbox file(s) that you want to open in Thunderbird, under the "Local Folders" directory. *
* Suggestion: Rename the mbox file(s) to a name that you can recognize when your open Thunderbird. For example if you have downloaded Gmail's "Sent" messages, you should have an MBOX file named "Sent.mbox". To easily recognize it in Thunderbird rename the MBOX file (e.g.) to "Gmail-Sent.mbox"
.
14. Open Thunderbird.
15. Under "Local Folders" click at the xxx.mbox file to view all the emails that it contains.
That's it! Let me know if this guide has helped you by leaving your comment about your experience. Please like and share this guide to help others.