To convert PST files to MBOX manually, you can follow these steps:

1. Export the contents of your PST file to a new Outlook data file (.pst) using Outlook's import/export wizard.
2. Once you have exported the data to a new PST file, close Outlook.
3. Download and install Mozilla Thunderbird if you don't already have it installed.
4. Open Thunderbird and create a new email account. If you already have an email account set up, skip this step.
5. Click on the "Tools" menu and select "Import".
6. Choose "Mail" and click "Next".
7. Select "Outlook" and click "Next".
8. Click the "Browse" button and select the PST file you exported in step 1.
9. Choose the folders you want to import and click "Next".
10. Select the Thunderbird folder where you want to store the imported emails and click "Next".
11. Click "Finish" to start the import process.
12. Once the import is complete, you should see all of your emails in Thunderbird.
13. To export the Thunderbird emails to MBOX format, right-click on the folder that you want to export and select "Save as".
14. Choose "File" as the format and select a location to save the exported file.
15. Choose a name for the exported file and add the ".mbox" extension to the end of the file name.
16. Click "Save" to export the folder to MBOX format.

Repeat steps 13 to 16 for each folder you want to export to MBOX format.

Regards,
Peter