The best option available online to convert Outlook PST file to MBOX format is EmailsGuru PST to MBOX Converter. It is specifically designed to convert PST files to MBOX format and offers users to easily access migrate their email data including emails, contacts, calendars, and attachments from Outlook to various MBOX supported email clients such as Thunderbird, Apple Mail, and others. It has simple interface, offering users batch conversion facility. Moreover, you can try free trial version as well.