To convert PST (Personal Storage Table) files to MBOX (Mailbox) format manually, you can follow these steps:

Install Mozilla Thunderbird: If you don't already have it, download and install Mozilla Thunderbird on your computer. Thunderbird is a free email client that supports the MBOX format.

Create a New Email Account: Launch Thunderbird and set up a new email account if you don't have one already. You can create a dummy account for the purpose of importing PST files.

Import PST File: In Thunderbird, go to "Tools" > "Import" > "ImportMailbox." Select "Outlook" from the options and click "Next." Choose the PST file you want to convert and click "Next" again. Thunderbird will import the emails from the PST file into your Thunderbird account.

Locate MBOX Files: Once the PST file is imported, Thunderbird saves the emails in MBOX format on your computer. Locate the MBOX files by navigating to your Thunderbird profile folder. The default location for the profile folder is:

Windows: C:\Users\AppData\Roaming\Thunderbird\Profiles.default
Mac: /Users//Library/Thunderbird/Profiles/.default
Linux: /home//.thunderbird/.default
Copy or Export MBOX Files: In the Thunderbird profile folder, you will find a folder named "Mail" or "Mail\Local Folders." This folder contains the MBOX files for your imported PST emails. You can copy these MBOX files to a desired location or directly export them to another email client that supports the MBOX format.

Regards,
Peter