I recently needed to open an MBOX file and found that you don't necessarily need a dedicated converter for it. The method depends on where the MBOX file came from.

Using Thunderbird:

Install and open Mozilla Thunderbird.
Create or find a local folder where you want to keep the MBOX emails.
Use an MBOX import add-on, such as ImportExportTools NG, to import the file.
Select the MBOX file from your computer.
Once the import is complete, open the folder and view the imported emails. You can check the message details, attachments, dates, and sender information.

Using Apple Mail on Mac:

Open Apple Mail.
Go to File > Import Mailboxes.
Select Files in mbox format.
Choose the MBOX file from your computer.
After the import is finished, open the newly created mailbox to view the messages and attachments.

If you only need to open an MBOX file occasionally and don't want to set up an email client, Kernel MBOX Viewer is another option. It lets you open and preview MBOX files directly, search through emails, and view attachments.

Learn More: https://www.nucleustechnologies.com/mbox-viewer-free.html