The easiest way to convert Outlook PST to MBOX is by using a professional PST to MBOX converter tool. Outlook does not provide any direct option to export PST files into MBOX, so manual methods are not available. You can use
Advik PST to MBOX Converter to convert your entire PST mailbox, including emails, attachments, and folders, into MBOX format. Just add the PST file, select the folders, choose MBOX as the output format, and start the conversion. This method is fast, safe, and works even if Outlook is not installed on your system.