The best way to convert Outlook PST files to MBOX format without using Microsoft Outlook is to using the converter application. PST files can be opened by this application and no Microsoft Outlook is necessary.

The solution that is mostly adopted by people is the PCInfoTools PST to MBOX Converter software tool. By using this software tool, one can easily convert their emails from PST file format into the MBOX file format.

The steps that should be performed are as follows:

* Install the converter application on the computer running Windows.
* Add your PST file into the application.
* Preview folders and emails inside
* Set saving format as MBOX
* Begin converting emails to MBOX

Reasons why this approach is effective:

* You don’t have to install Microsoft Outlook;
* All email attachments and formats will be saved.
* Works even with large size PST files;
* Good for transferring emails from Outlook to Thunderbird.

It's the common practice adopted by individuals who no longer have MS Outlook installed on their systems but have PST files backed up.